Some watercolor paints are sedimentary, which means there are particles within the paint's pigments when used in a wet wash they will sink to the bottom of the pool of water even on your paper. This may cause a granulating textured effect when dry.
In the example shown here, I am using MaimeriBlu watercolor paints, Burnt Sienna (puddle on the top left) and Ultramarine Deep (puddle on the top right). The center puddle is a mixture of the Burnt Sienna and the Ultramarine Deep. This creates a nice gray color when mixed evenly. If you want it to lean a bit more to the blue side, add more Ultramarine Deep. If you want it to lean more to the brown side, add more Burnt Sienna. This mixture also has the ability to go very dark and create a nice rich black.
Sample 1: A mixture of the two colors mixed completely on the palette and applied to dry paper.
Sample 2: I applied a juicy wet Burnt Sienna to the paper and then added wet Ultramarine Deep and mixed it directly on the paper. Allowed to dry naturally.
Notice how even the color is when first mixed on the palette and then notice how you can obtain a more varied watercolor effect by mixing the colors right on the paper.
Sample 3: I applied a small square of water to wet the paper first, then I painted on the premixed color.
Sample 4: Again, I applied a square of clean water to wet the paper first. I added in the Burnt Sienna and then added the Ultramarine Deep and mixed them together on the wet paper.
Having wet the paper first, you will be giving the sedimentary colors more water to float around in. The different particles will sink to the bottom at different rates.
Allow your sedimentary and granulating colors to dry naturally on their own. Do not speed up the drying process with a hair dryer. If you allow these colors to dry on their own, they will work their own magic and create a fabulous texture.
These two colors are both sedimentary and create a beautiful granulating effect. Burnt Sienna and Ultramarine Deep are great color combination for creating rocks, trees, stormy skies.