Fiberglass boards are available in several colors.  Sitting right
here, I have unetched board stock that is white, blue, green and
yellow respectively.  I've also seen black boards.

Solder mask is available in a variety of colors too.  Depending on
the supplier, the color helps indentify the material.

Don't forget conformal coating.

I don't think I've seen phenolic impregnated paper boards in
anything other than the familar brown, but it would not be
difficult to add a dye to change the color.
