thanks for the advice, but as it turns out, my problem wasn't solved by just using digitalWrite - it works in the given example, but digital write does just do HIGH or LOW (0 or >0), so i still cant mix any colors on the leds steplessly 
EDIT: @Oracle: I'll try out your idea as well, but i would really like to mix as much colors as possible... Maybe i'll better get some of these LED controller-thingys (TTLC5940), but i have no idea how to use them or where to get them cheap in germany...