Hi, I have just upgraded my bulbs to incorporate an ATI purple plus and a pro colour and really like the colours.
Mine is a 6 bulb unit though and my order is:
Aqua blue
10k white
Pro colour
10k white
Purple plus
Aqua blue
This is from back to front. You could do something similar with a 4 bulb but I would only use the pp as the pro colour is a little pink.
Looks a little cartoon like if you get what I mean.
Hth.