You can get other colors from professional board houses, but you usually need to order lots of boards and pay a premium. I've never seen blanks other than brown or green. Some small order 'batchers' can use other colors. Dorkbot will make boards in purple http://dorkbotpdx.org/wiki/pcb_order
Anything else will require some experimentation:
Dip a light brown board in die after etching and stain the core.
Use the lightest brown, single sided board you can find and paint or die the back side. It might "bleed through"
Paint the board after etching and carefully sand with fine grain sand-paper on a flat surface (glass, ceramic tile) until the paint is removed from the higher copper, but not the lower core.
Color in all the open spaces with a permanent marker or paint pen.
Black-out the surface the same way you do etch resist. Laser print a "layout" with black in all the open areas (or print the inverse layout of the etch resist design) and iron it to the board after etching. Could you use a color laser printer?.
Spray paint the board after it has been populated and tested. I've sprayed boards flat black to look for hot spots with a thermal camera.
Make your own circuit board blanks. Glue thin copper sheets to whatever insulating substrate you want then etch normally. You may need to experiment to find the right substrate and/or glue that isn't damaged or stained by the etchant. You could do some really cool things with colored glass or plexiglass.
Draw simple circuits with conductive ink or use point-to-point wiring on your choice of substrate.
Good luck and if you find something, let us know.