Have you thought of using a marblising tool? Basically, you do the following;
1) Paint your chosen base colour over the nail.
2) place a blob of nail polish in the centre of the nail and another blob of a different colour in the centre of that.
3) Using the marblising tool you pull the paint lightly from the centre of the nail towards the edges. (as though you are making a star shape)
You can use as many colours as you choose and make into you chosen shape.
Unfortunatley I don't have pictures to back this up with but I'm sure somebody else will, and maybe able to explain it better.