I apologize for the delayed response - it has been a very busy week internally at Cesium. Did you get a chance to check out the demo that I shared in my last response? It showcases how to add a single color-accurate polygon to the cesium viewer.
In general, color accuracy and material accuracy in 2D has been a pain point for our community. The following thread provides a nice summary of our findings and links to a lot of important resources.
Let me know if you have any other questions or concerns.
-Sam