Harlequins are a painters dream, and there are sooooo many ways to paint them. I painted a set years ago, but I was unadventurous and simply copied the GW scheme.
One very interesting idea that I heard many moons ago on this very forum was to paint a troupe of ten harlequins. Seven of these would have each be painted primarily with one of the seven primary colours. The shadowseer would be painted white, the death-jester black and the troupe-master some crazy mix of all colours. Note that this was way back in 4th edition, so the make-up of the squad may now be different, but I'm not a gamer anymore
Beyond that, you could try using techniques such as wet blending or marbling. What about a scheme that begins with black at the feet and works its way to red or blue towards the head? If you have an air-brush, this can really help with these techniques.
One good bit of advice that I can offer is that the masks look awesome if painted in a very striking colour. If you use fairly dark colours for your holo-suits, the masks will pop out if painted white. You could even try a metallic finish.
The sky is the limit!