The answer to the question "What color is watercolor paper?" is not a simple "White, of course." The photo above shows this very clearly -- both pieces of paper are watercolor paper, yet definitely not the same 'white'.
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, blueish 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 painting 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.)
