CARDBOARD.
Your own judgment should be sufficient to know good cardboard. The more you pay the better goods you get, that is, if you deal with a reputable house.
Use a pure white, heavy coated board from 6 to 10 ply, according to the size of the sign you are making. See that your board is of sufficient thickness to prevent warping. If you use frames, of course your board need not be so heavy.
Colored card-boards are well enough, occasionally, but as a rule use plain white; people do not tire of it so easily.