Thanks for reaching out to us, I am sorry we are using only blue for the polygon color. I will forward your feedback to product team for future release.
One workaround may help with your scenario is, you can change the brightness to "-100" when you draw and revert the brightness back when you done as below screenshot. This will help to make things clear.
Hope this helps and thanks for the feedback again.
Regards,
Yutong
-Please kindly accept the answer if you feel helpful, thanks.