Painting Wet-on-Dry:
If you want sharp edges to what you're painting, then any paint already put down on the paper must be dry before you paint another shape. If it is completely dry, then the shape will stay exactly as you'd painted it. If it isn't completely dry, the new layer will diffuse into the first; this is done deliberately when you're painting wet-on-wet.

Painting Wet-on-Wet:
Adding paint to a wet layer of paint on the paper produces a soft, diffused look as the colours mix. The extent to which the two colours mix depends on how wet the first layer still was and how dilute the second colour was. You can get anything from a soft-edged shape to a widely spread pattern. In the example here, the blue was slightly damp when the red stripe was added, so the red hasn't mixed very far into the blue.

Being able to predict the results you're going to get working wet-on-wet takes practise, but as this technique can produce spectacular, lively paintings it's well worth experimenting with it. It's particularly useful for suggesting movement in a painting and for diffusing shapes when you don't want too much detail. Make up a file of your various attempts with notes on the colours you used (some pigments collect on the paper's surface, creating more of a texture than others), how dilute the second colour you added was, how wet the first layer was, and what paper you used.

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