@kdfrawg None taken — I posted the question because I don't consider my vocabularly nearly large enough to be sure.
Yellow, ending in "w", is a consonant — for that, I'll refer you to the original criteria "doesn't end in a consonant, y, or e".
One other issue is finding out whether there is such a thing as an adjective that has always been an adjective, i.e. not derived from a noun. "Black", for example, often referred to ink as well as the colour itself; for example, if I remember correctly, "orange" is derived from a noun ("naranga" in Sanskrit).