In css-mode color strings get a background color corresponding to the respective color - like this (image):
How to achieve this behaviour in other modes?
OK… wasn't aware of rainbow-mode, wich does exactly, what I intended. Of cource I found it after I posted question here ^^.
So the solution is:
M-x package-install RET rainbow-mode RET
To answer my question completely and enable it in all modes: I added
(define-globalized-minor-mode global-rainbow-mode rainbow-mode (lambda () (rainbow-mode 1)))
to my init file.