There are many things to consider when choosing paint: color, tone, tint, and opacity. Each is important, yet how opaque a paint is among the biggest concerns for painters.
Different paints will have different opacities and they vary greatly by pigment, formulation, and manufacturer. You will find that the more opaque a paint is, the better it is for covering what is underneath it and that will play a factor in hiding mistakes and creating glazes for your paintings.
A paint color is said to be opaque when it hides what's underneath it. When you can't see any or much of what's beneath the color, it is an opaque paint. If you can see an underpainting, then that paint is just the opposite of opaque, it is transparent.
The science behind the opacity of paints can get complicated, but there are two important factors:
Any color in the spectrum can be either opaque, transparent, or anywhere in between. For instance, titanium white is known to be very opaque and that is why it's perfect for covering up painting mistakes. Zinc white, on the other hand, is semi-opaque to transparent (depending on the brand) and is a good candidate for glazes.
Tip: It is important to understand that opaque does not mean white.
Some pigments are extremely opaque. Popular among those are titanium white and cadmium red. Many of the paints that include cadmium or cobalt in the name are opaque, though there are many other opaque pigments.
The opacity of a certain color will also vary by manufacturer. Many artists find that one brand of cadmium red is more opaque than another brand of the same color. Also, professional artist-grade paints tend to be more opaque or have a more fine-tuned opacity rating than beginner or student paints.
If the opacity of a paint can vary so much from pigment and brand, how can you tell the opacity of a particular paint? Your answer lies in labels, research, and testing.
The label of the paint tube should have an indication of whether that color is opaque or not. Cheaper brands sometimes lack this information but many paint manufacturers understand its importance to artists.
How opacity is indicated on the label may vary:
Should all of those resources fail or you wish to test the opacity of a paint you mixed yourself, there is an easy way to discover the opacity of any paint you are using.
Through the use of other paints and mediums, you can change the opacity of your paint and make it more or less opaque. The degree of success for your intention may vary, but it is worth trying and working with until you get the desired results.
To make an opaque paint more transparent: Add a medium designed for the type of paint (acrylic, oil, etc.) that you are working with until it is as transparent as you like.
To make a transparent paint more opaque: Mix it with an opaque paint such as titanium white or carbon black. Do be aware that there will be a color shift, so you will have to work with it to get a color you like. You can also use an opaque paint of the same color to make transparent paints more opaque (e.g., use cadmium red to add opacity transparent red paints).
It should be noted that it is easier to make an opaque paint more transparent if it is already semi-opaque. Going back to our white example, you will find that zinc white will become more transparent with less mixing than titanium white. The exact opposite is true when trying to make transparent colors more opaque.