Welcome to the community! 
Thank you for sharing your sandcastle demos.
Regarding your first question, I suggest you check out the following community forum thread. I had a great conversation with @11179 where we discussed color accuracy in 2D. In particular, be sure to look at the second half of the thread.
Regarding your second question, what material changes do you notice when you set the height attribute of your polygon? I have not encountered this issue in the past and was not able to reproduce the bug on my machine. Does anyone from the rest of the community have any suggestions?
Please let me know if you have any other questions or concerns. I am looking forward to learning more about your application.
-Sam