For this demo I used candy clay. I started with red and green and rolled them out fairly thin.
Then I layered the two colors and rolled them out again so that it was green on one side, red on the other, but you could see the two colors starting to meld together.
Next I cut out some leaves using a rose leaf cutter. These will be red on one side, green on the other.
I put the leaf in a rose leaf press and made the impression of the veins. I pressed fairly hard, so that the red color would show through a little on the green side.
The veins will show a little of the darker red color, and will give the leaf a more dimensional look, as opposed to being one flat color.
You can reroll the clay and cut out more variegated leaves, but be careful not to combine the colors too much. Leaving them uncombined will give a more mottled look and will look more like real leaves.
The same technique was used to make the roses on this wedding cake: http://www.youtube.com/watch?v=EpEP-sQGAVI