Thanks very much, Lumpy.
I did what you said, and found that the problem was solved when I uploaded a new version of "templates/yourstyle(for me subBlack)/posting_body.tpl"
Now I have to compare code to see if I can find where the difference is. I have mods installed, so it is unfortunately not a case of simply keeping the replacement file. But at least I know which file is faulty. Thanks very much!
[edit: ok, I have had a hard look at the code, and can't see what is causing the color tags to not work.
There must be an error SOMEWHERE though. This is the file.]