(Edit: I did not realise that your question is about a rich text edit column on a datawindow. What's described below is for a rte control. But ... the good news is that the CTRL+ALT+D key combination also works for datawindows. Just click somewhere in the richtext column's text (focus must not be on that annoying toolbar that pops up) and hit CTRL+ALT+D). I believe that in previous version this key combination was different, something like Ctrl+D, but I'm not sure). (Also: use the Alt key on the left of the keyboard, not the "Alt Gr" on on the right).
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What follows below is for the rte control:
You can change the colour, it's just not become easier over the years (probably because the people who design this are morons. The control was way better in the 90-ties).
1. First, select some text
2. then do a right-mouse-button click
3. and select properties
Then go to the second tab page:
If you use CTRL+ALT+D, you'll get a different dialog and you don't have to select any text first:
Regards