I spent some time reading. Additive colour printing uses the sharp cutting filters. I guess #29,47b and 59? It wouldn't suprise me if the combination of red,blue and green leds with some sort of filter in the head would work. The same thing could be used for high contrast with sticking a blue filter in the head.
Did Huw use 1w leds then? Makes me feel a little better-)) Those 5w ones are expensive if you need 200 of them. Plus the cost of a heat sink.