On further testing, I found that if we apply any dark Theme/Style the font color of the text in Editor gets set to White color do what you may to change the color of text it does not change!
I also had the same issue recently and I tried to figure out what caused it. And it turned out to be quite simple: when you set the TIERichEdit component's 'StyleElements' value of 'seFont' to 'false' then all colours will be allowed again. It happily worked for me and it surely made my day.