They could also work with the opposite colors of already existing civs, just like Japan and Austria, France and Greece, Egypt and Rome, Songhai and Mongolia, England and The Huns, Mayans and Iroquois, Persia and Siam, etc. Not saying they are exactly the perfect opposite scheme, but they are very simillar to it.
Ex.: Purple and Blue (byzantium)
Light blue and Green (celts)
Yellow and Blue (sweden)
Purple and White (carthage)
White and Dark Blue (america)
Red and Green (ethiopia)
Also, there are colors they used in scenarios that would work. Assyria's colors is basically the same of Ayyubids from Into the Renaissance. Of course, IF it is 100% sure Assyria. But they could use the color scheme from the Manchu (Korea scenario), the Goths (Fall of Rome), the Franks (Fall of Rome), Hittites (WOTAW), Summeria (WOTAW), Almohads (Into the Renaissance), England on 1066, dark blue/orange from Polynesian scenario, etc etc etc. Many options there, actually.