Hi Tibor,
the easiest way to do it without having to go into fusion is to do it on the edit page:
First, generate a text in the Effects library (under Titles-->Text).
After that you go to the "Generators" panel right underneath and generate a "solid color".
Crop the "solid color" in the "inspector" menu to your liking and place it underneath your text (don't forget to do it on the timeline as well). You can even do a simple keyframe animation of the "solid color".
Hope this answers your question,
Cheers,
Dawid