in reply to Tk Tablematrix set color of selected row
I see you've used the tagConfigure() method here (and twice in your follow-up post). I'd say you're on the right track with that; however, you probably want the 'sel' tag. See the Tags section of the Tk::TableMatrix documentation.
"... set the color ... when a cell/row is selected/in focus/highlighted (I do not know how to express it, sorry)."
I can understand that it may be difficult to use the exact terms for something that you're unfamiliar with; unfortunately, that becomes a problem when trying to provide an answer.
I think Tags is what you want. There are other options that you may find useful. Those listed under "STANDARD OPTIONS", such as -highlightbackground and -highlightcolor may serve your purpose (see Tk::options for their documentation). Also, the Tk::TableMatrix option -invertselected might be sufficient for your needs.
If you read through the documentation, you'll probably find what you want; in addition, by doing this, you'll learn the terminology. As an extra bonus, tags are used by other widgets so you'll also learn principles you can use elsewhere, e.g. Tk::Text; the same applies to other areas such as Indices.