I am using Moodle 3.5.2, the standard Boost theme, ATTO, and have the morefontcolors add-in installed. I see it as shown below.
I wonder if you might be facing an ATTO versus TinyMCE editor issue.
Well, here's the interesting thing. I use discussion forums a lot in my courses, and for many years. Seldom do either I or students use different font colors. However, I thought that it would be good to provide more colors, and would still add these. It is just interesting that change font colors is seldom used.