You can use the food color spray in a can. Wilton has a whole line and also just came out with pearl.
You can see them here:
You can also try and use luster dust with a dry brush directly on the fondant. That works well.
You could probably use the Wilton color mists but I don't know how much they are and that might get expensive if you have to buy all the colors. Another idea would be to use fondant in those colors and marble them together to get a swirly kind of look. That's all I can think of without airbrushing the cake. Pretty cool cake though.
I used the luster spray from global sugar art for this cake, it's a little pricey, but I didn't have an air brush machine either. It works perfectly.
In the link that I am posting, the person diluted some color and use a sponge to paint her cake.
In another place in the inter-net the person use one of those plastic spraying bottles( the ones they use in the hair salon) she put vodka and edible colors, shake it and sprayed it on the cake. She gave the proportions of vodka and color but I can not find it, sorry
Well, anyway... I hope it is of some help