So now, I have one color always set to the maximum: that one is always on at maximum brightness. The other two can be looped through normally. So, I have 3 * 7 * 7 - 2 (because the 7,7,7 light occurs three times!) = 145 different colors. Not spectacular, but good enough.
Friday I have the day off (compulsory day off at the office, because of the end of the four day's marches here in Nijmegen), I might make a small plexiglass plate to mount the RGB-LED behind, so that I can make pictures of the different colors.
If someone is interested in the code, give me a holler and I'll post it somewhere!