The most important thing in watercolors is the texture of the paper.
They use a texture for sea, another for land, and so on. If you look at the see you will see how the sea and land repeats itself. You will need to make a seamless texture that you can repeat ad infinitum. Those are made with filters on photoshop or GIMP, or you could crreate them using fractals(very difficult for it to feel like a realistic watercolor, that is the reason they did not used them).
I don't like GIMP for that, plugins had a very confusing syntax, I prefer ImageMagick or better I use my own programs. Using Photoshop you can do it "with your hand"(selecting a vector shape, grow it with border transition, apply texture, and repeat), record and then use the macro for all your vectors.
I would use an OpenCL or CUDA program, faster and way more control.
They use a texture for sea, another for land, and so on. If you look at the see you will see how the sea and land repeats itself. You will need to make a seamless texture that you can repeat ad infinitum. Those are made with filters on photoshop or GIMP, or you could crreate them using fractals(very difficult for it to feel like a realistic watercolor, that is the reason they did not used them).
I don't like GIMP for that, plugins had a very confusing syntax, I prefer ImageMagick or better I use my own programs. Using Photoshop you can do it "with your hand"(selecting a vector shape, grow it with border transition, apply texture, and repeat), record and then use the macro for all your vectors.
I would use an OpenCL or CUDA program, faster and way more control.