David
I tried it, but it does not give very good results with some combinations (some colors like orange, yellow). I had better results testing two colors at a time:

retcol = 16777215 && default white (rgb(255,255,255))
IF (red > 128 and green >128) OR (red > 128 and blue >128) OR (blue > 128 and green >128)
retcol = 0 && black
endif
RETURN retcol

Thanks!