1. Take two colours, whatever. I have red and orange here.
2. Take one of the two colours and lower the opacity right down to 15.
3. Draw using this colour go over the other colour. Select that colour, then undo the scribble.
4. Using this new colour, draw over the joining point of the two colours.
5. Once again, using low opacity, draw over the other colour, but it gets tricky here, you must do this twice. First time you go over the yellow and draw the line down the yellow side of the joining line, then draw over the red, and take that colour to go over the joining line on the red side. Hope that makes sense. x.x
6. Repeat this until you're satisfied with the smoothness.
7. Take blur tool and go over, again, until you're satisfied.

There you go christiangirl, and any one else interested.
Just a cheap quicky, i'm sure you can all do better.