Thanks Jon, that worked. Certainly not obvious. I had been beginning to wonder if there was some program adversely affected DOPUS, but that seemed less likely here.
Suggestion:
Have an informative prompt:
'If you wish to appy a background tab colour, Visual Styles (explanation) will be disabled. Do you wish to proceed?'
rather than relying on the user to know two not obviously related options must be changed.
Yes we've thought that before (I think Leo actually suggested moving the 'Use visual styles' option to the Display / Colors & Fonts page for Folder Tabs.)