Hi,
If I understand your concern correctly, I faced a similar question a few weeks ago.
My research showed that the grouping heading line cannot be customised in colour. What I did do, however, was to work in combination with conditional formatting for the data rows. Here you would have the possibility, for example, to assign a specific background colour to a row, or only to a single cell per row. The best thing to do is to try it by adding an additional column and configure the corresponding rules via "advanced options". Then you can switch to text mode and have a good template with which you can continue to work.
Regards
Lars