How could you test whether color influences the rate of warming in an animal?
The color of a surface is one of the factors that affects the surface's albedo, a property which helps describe how a surface reflects and/or absorbs radiation that hits it.
To test this, you might obtain a piece of light-colored leather or fur (real would be preferable) and cut it into strips. Keep one strip in its natural light color, then dye the strips in a natural tone (brown or gray) progressively darker so that you end up with a series of strips from light to very dark. Set the strips in in a row in direct sun or under a heat lamp, with a thermometer underneath each strip. Monitor the temperature increase beneath each strip; you can calculate the rate as degrees of increase per minute.