Hi Wolfgang,
I am not sure what you want to do.
Your profile shows Mac OS X 10.4.10. If that is up to date, you are using either Pages '08 or Pages '09. Or if your OSX is Mavericks 10.9.1, perhaps you are using Numbers 3.x. Please tell. 🙂
I do not know how to set a highlight (background) colour for single words in a cell in Numbers.
You can set the Fill colour for a whole cell.
When you say "highlight" do you mean the text colour? Or do you mean Bold?
When you try to highlight some words, which option is gray? Which Toolbar button or which Menu item is gray?
More information will help to find a solution.
Regards,
Ian.