Background by User-ID in list and profile view

Is it possible to set (for example) a blue background for User 203 and a red background for User 207? This settings should be used in any list and profile view!

Profiles don't have a class that's user id specific. Same for userlist rows. You could use CB Auto Actions to add CSS to a user profile and check for user id in the conditional, but this usage isn't going to work for userlists.

I had found an easy workaround:
New Field (field type = Drop down (Single Select)) maybe "Background-Color" with all colors you need!
This small Script in components\com_comprofiler\plugin\templates\default\default.php:

instead of

in line 496!

Now every user can choose the color he want in his profile edit area!

That can be done using CB Auto Actions without having to edit a single line of core code. That also will be completely invalid in next CB release as the templates have been rewritten. Using CB Auto Actions you'll use a Code action on the before profile display trigger then set method to CSS and supply your CSS there (there's a class surrounding the entire CB profile).

This is only a little problem. I can override the template after every update. But you write in your first message "CB Auto Actions will NOT work for userlists"?

CB Auto Actions will work fine on userlists, but userlists rows are not unique per user id. They are unique by row index. So for example the first row is #cbU0.

You could use jQuery in a code action to alter the DOM and add to the rows as needed. For example to do this you'd add the color field to the userlist so it outputs the value as a field. You'd then use jQuery to grab the fields value (each field is surrounded by a unique id) looping through each row then adding the value to the row as CSS then removing the field from display.