In color mixing for painting, the fundamental rule is that there are three colors that cannot be made by mixing other colors together. These three, red, blue, and yellow, are known as the primary colors.
If you mix two primaries together, you create what is called a secondary color. Mixing blue and red creates purple; red and yellow make orange; yellow and blue make green. The exact hue of the secondary color you've mixed depends on which red, blue, or yellow you use and the proportions in which you mix them. If you mix three primary colors together, you get a tertiary color.
What About Black and White?
Black and white can also not be made by mixing together other colors, but as they aren't used in color mixing to create colors, they get excluded from color mixing theory. If you add white to a color you lighten it and if you add black you darken it (though some painters don't use black at all, see Color Mixing Lesson: Black and White).
Yes, you can buy various different blues, reds, and yellows. For example, blues include cobalt blue, cerulean blue, ultramarine, monestial blue, and Prussian blue. Reds include alizarin crimson or cadmium red, and yellows cadmium yellow medium, cadmium yellow light, or lemon yellow. These are all primary colors, just different versions.
It's not a question of there being a right or wrong primary to use, but rather that each blue, red, and yellow is different and produces a different result when mixed. Each pair of primaries will produce something different, sometimes only subtly different.
It's color mixing at its most basic, the first step on a journey with color.
Every color has a certain bias towards what's called warm and cool. It's not something that's overwhelming; it's subtle. But it's an important element in color mixing as it influences the results.
As a group, reds, and yellows are considered warm colors and blue a cool color. But if you compare different reds (or yellows or blues), you'll see that there are warm and cool versions of each of these colors (relative to each other only). For example, cadmium red is definitely warmer than alizarin crimson (though alizarin crimson will always be warmer than, say, a blue).
It's important to recognize that individual colors have a bias towards cool or warm for color mixing. If you mix two warms together, you'll get a warm secondary color and, conversely, if you mix two cools together you'll get a cool secondary.
For example, mixing cadmium yellow and cadmium red light creates a warm orange. If you mix lemon yellow with alizarin crimson, you get a cooler, more gray orange. Mixing secondary colors is not only about the proportions in which you mix two primary colors, but also knowing what different reds, yellows, and blues produce.
Secondary colors are made by mixing two primary colors together: red and yellow to get orange, yellow and blue to get green, or red and blue to get purple. The secondary color you get depends on the proportions in which you mix the two primaries. If you mix three primary colors together, you get a tertiary color. Secondary colors are made by mixing two primary colors together. Red and yellow make orange; red and blue make purple; yellow and blue make green.
Red and yellow always make some kind of orange, yellow and blue a green, and blue and red a purple. The actual color you get depends on which primary you're using (for example whether it's Prussian blue or ultramarine you're mixing with cadmium red) and the proportions in which you mix the two primaries. Paint a color chart where you record which two colors you mixed and the (approximate) proportions of each. This will provide you with a ready reference until you get to the stage when you instinctively know what you'll get.
The proportions in which you mix the two primaries are important. If you add more of one than the other, the secondary color will reflect this. For example, if you add more red than yellow, you end up with a strong, reddish orange; if you add more yellow than red, you produce a yellowish orange. Experiment with all the colors you have - and keep a record of what you've done.
Color mixing gives you a range of colors with a minimum number of tubes of paint (very useful when painting outside your studio). If you're using a lot of a certain color, you'll probably decide it's easier to buy it in a tube rather than mix it up again and again.
But you'll find that there'll always be an instance when the color you want simply doesn't come ready-made, such as a particular green in a landscape. Your knowledge of color mixing will enable you to adapt a ready-made green to the shade you require.
The advantage of buying a premixed color is that you are assured of getting the identical hue each time. And some single-pigment secondary colors, such as cadmium orange, have an intensity that's hard to match from mixed colors.
Browns and grays contain all three primary colors. They're created by mixing either all three primary colors or a primary and secondary color (secondary colors being made from two primaries). By varying the proportions of the colors you're mixing, you create the different tertiary colors.
Mix a primary color with its complementary color. So add orange to blue, purple to yellow, or green to red. Each of these makes a different brown, so once again make up a color chart to give you a quick reference to refer to.
Mix some orange (or yellow and red) with a blue then add some white. You'll always want more blue than orange but experiment with the amount of white you use. You can also mix blue with an earth color, such as raw umber or burnt sienna. With watercolor you don't have white paint; to lighten a gray you add more water instead of white, but remember the gray will be lighter when it dries.
If you mix too many colors together, you'll get mud. If your gray or brown isn't coming out the way you want it to, rather start again than add more color in the hope it'll work.
The complementary color of a primary color (red, blue, or yellow) is the color you get by mixing the other two primary colors. So the complementary color of red is green, of blue is orange, and of yellow is purple.
The complementary of a secondary color is the primary color that wasn't used to make it. So the complementary color of green is red, of orange is blue, and of purple is yellow.
When placed next to each other, complementary colors make each other appear brighter, more intense. The shadow of an object will also contain its complementary color, for example, the shadow of a green apple will contain some red.
The color triangle as (shown above) makes it easy to remember: the three primary colors are in the corners. The color you get by mixing two primaries is between them (red and yellow make orange; red and blue make purple; yellow and blue make green). The complementary color of a primary color is the color opposite it (green is the complementary of red, orange for blue, and purple for yellow).
It may seem like a simple exercise, hardly worth spending time on, but it's the first step in a fundamental painting skill -- successful color mixing. Put it up on the wall where you can see it at a glance until you've internalized which colors are primaries, secondaries, tertiaries, and complementaries.
If you mix complementary colors with one another, you get a tertiary color, particularly browns (rather than grays).
While it may seem logical that to lighten a color you add white to it and that to darken it you add black, this is an oversimplification. White reduces brightness so although it makes a color lighter, it removes its vibrancy. Black doesn't so much add darkness as create murkiness (though there are instances in which black is uniquely useful, such as the range of greens it can produce when mixed with yellow!).
Adding white to a color produces a tint of that color, makes a transparent color (such as ultramarine) opaque and cools the color. This is most noticeable with red, which changes from a warm red into a cool pink when you use titanium white. You can add white to lighten a color, but because this removes the vibrancy of color you'll end up with a washed-out picture if you use white to lighten all your colors. Rather develop your color mixing skills to produce hues of varying intensity. For example, to lighten a red, add some yellow instead than white (or try zinc white). Watercolor paints are, of course, transparent, so to lighten you simply add more water to paint to let the white of the paper shine through.
Black tends to dirty colors rather than simply darken them. Of the most common blacks, Mars black is the blackest and is very opaque, ivory black has a brown undertone, and lamp black a blue undertone.
Think about how much is truly black in nature. Shadows are not simply black nor a darker version of the color of the object. They contain the complementary color of the object.
Take, for example, the shadow on a yellow object. If you mix black and yellow, you get an unattractive olive green. Instead of using this for the shadow, use a deep purple. Purple being the complementary color of yellow, both will look more vibrant. If you can't figure out what colors are in the shadows, simplify what you're looking at by placing your hand or a piece of white paper next to the bit you're having trouble with, then look again.
At various times in their careers, the Impressionists didn't use black at all (find out what they used instead). Take Monet's paintings of Rouen Cathedral in the morning full sunlight, in dull weather, and in blue and gold to see what a genius can do with shadows (he did 20 paintings of the cathedral at different times of the day). It's not true to say the Impressionists never ever used black, but they certainly popularized the idea.
If you can't see yourself working without black, then consider mixing up a chromatic black rather than using a straight-from-the-tube black. It also has the advantage not 'killing' a color it's mixed with to the same extent.
Different pigments have different covering properties. Some are extremely transparent, barely showing on top of another color. Others are extremely opaque, hiding what's beneath. Considering this, and not just what the color is, can enhance a subject. For example, using a transparent blue in a sky gives a greater feeling of airiness than an opaque blue will. Compiling a chart of the colors you regularly use, such as the one above, shows at a glance how transparent or opaque a color is.