The color of watercolor paper varies from manufacturer to manufacturer and even between different types of paper made by the same manufacturer. Watercolor color can range from a warm, rich cream to a cold, bluish white. Descriptive names for watercolor paper colors include traditional, extra white, bright white, and absolute white. The difference can be easy to see, or it can be slight, hardly evident even when you've two different sheets of watercolor next to one another.
The important thing is to be aware that the color of watercolor paper does differ, and does have an impact on your painting. A watercolor paper with a cream color can make your colors appear muddy. A watercolor with a blueish bias can give yellows a greenish appearance. (But if you're using a lot of graphite in a painting, a creamier paper can be more appealing to the eye than an intense white paper which can glare too much and be hard on the eye.)
When you're buying watercolor paper, take its color into consideration just as you would its finish and weight.
Note for Beginners: If you've only just started using watercolors, don't stress too much over the color of your watercolor paper. The important thing is to be aware that it differs, to try various brands and weights to see what each is like. Don't buy only one brand and never try anything else.
A watermark is watercolor paper's equivalent to the sewn-in label in a piece of clothing—it tells you who made it. Depending on the manufacturer, it may also tell you more, such as the brand and the cotton content.
The watermark in the photo above, for example, tells you not only that this sheet of paper is manufactured by Fabriano, but that it's a sheet of Artistico. Fabriano is said to be the first company to use watermarks, starting towards the end of the 13th century.)
Watermarks are most easily seen by holding a sheet of watercolor paper up to the light. A watermark can be added either by it being part of the screen used for making the paper (it shows up because less paper pulp is used in this area) or by it being embossed (indented) onto the paper when it is still wet.
Incidentally, holding a sheet of watercolor paper so the watermark reads correctly doesn't mean you have the "right" side of the paper facing towards you. How it's done differs between manufacturers. Neither is the absence of a watermark a sign that it's a cheap and nasty piece of watercolor paper.
There is a difference between the two sides of a sheet of watercolor paper, with one side usually slightly smoother (less hairy) than the other. But we wouldn't them "right" and "wrong" because which was which would depend on what you require from your watercolor paper.
The smoother side of a paper is better if you're painting a lot of detail, while the hairier side is better if you're wanting to build up color by using use lots of glazes.
A deckle edge on a sheet of watercolor paper is an uneven or frayed edge. It's the natural edge that is formed when paper is made, where the paper pulp thins out at the edges.
A full sheet of handmade paper usually has deckle edges on all four sides. A sheet that has been cut will have one or more straight edges, depending on how it was cut. Some machine-made papers have simulated or 'artificial' deckle edges.
The width of a deckle edge varies from manufacturer to manufacturer. On some papers it is quite narrow; in others, it is quite wide and intended as a decorative edge to the sheet. Some artists like to keep a deckle edge and to frame a watercolor painting so it shows; others trim it off. It's a matter of personal preference.
Watercolor paper is divided into three categories according to the surface of the paper: rough, hot-pressed (HP), and cold-pressed (NOT).
As you'd expect from the name, rough watercolor paper has the most textured surface or most prominent tooth. It's sometimes described as having a pebbly surface, a series of irregular rounded shapes like a pebble beach. On rough paper the paint from very watery washes tends to collect in the indentations in the paper, creating a grainy effect when the paint dries. Alternately, if you whisk a dry brush lightly across the surfaces, you'll apply paint only to part of the paper, the tops of the ridges and not in the indentations. Rough paper is generally not regarded as a good paper for painting fine detail but is excellent for a loose, expressive style of painting.
Hot-pressed watercolor paper has a smooth surface with almost no tooth. Its smooth surface is ideal for painting fine detail and for even washes of color. Beginners sometimes have problems with the paint sliding around on the smooth surface.
Cold-pressed watercolor paper is sometimes called NOT paper (as in not hot pressed). It's the paper in between rough and hot-pressed paper, having a slightly textured surface. Cold-pressed is the most commonly used watercolor paper surface as it allows for a good amount of detail while also having some texture to it.
The soft-pressed watercolor paper is in between hot-pressed and cold-pressed, with a slight tooth. It tends to be very absorbent, sucking in the paint, making it harder to paint dark or intense colors.
Once again it's important to remember that surfaces vary from manufacturer to manufacturer.
The thickness of a sheet of watercolor paper is measured by weight. So, logically, the greater the weight, the thicker the sheet. It is measured either in pounds per ream (lb) or grams per square meter (gsm). The standard weights of paper are 90 lb (190 gsm), 140 lb (300 gsm), 260 lb (356 gsm), and 300 lb (638 gsm).
Thinner paper needs to be stretched to prevent it from buckling or warping when you paint on it. How thick the paper needs to be before you can happily paint away on it without buckling does depend on how wet you tend to make the paper as you paint. Experiment with different weights to see, though it's likely you'll find that paper less than 260 lb (356 gsm) wants to be stretched.
Not having to stretch it is not the only reason for using heavier paper. It'll also stand up to more abuse, and take a greater number of glazes.
Watercolor paper is also sold in blocks that are 'stuck together' at the edges. This format has the advantage that the paper doesn't have to be stretched before you paint on it to avoid it buckling.
There are disadvantages to watercolor block though. For starters, you have to leave the painting to dry in the block (if you separate a sheet off before it's dry, it may buckle as it dries). Which means that you need more than one block if you want to do several paintings one after another.
Also, some manufacturers don't assemble their blocks so that the same side of the paper is always at the top. So you may find yourself painting on the 'right' and then the 'wrong' side of a paper. And some artists say that paper in a block didn't have a surface texture identical to the same brand in a single sheet, so watch out for that.
Watercolor paper sold in blocks is usually more expensive than any other format, but the convenience may make you decide it's worth it.