This article was originally written for Fictorians.com and was pitched at people building their own worlds for novel writing, but the general process should be useful for a broad variety of world builders.
Let’s get one thing out of the way right now.
A map shouldn’t be pretty.
Blend modes are a wonderful feature of Photoshop, and also appear in many other programs, including Gimp. Here’s a few I use regularly. I’ve taken the same styles o text and shown how they appear using the different blend modes. Further down, you can see the effect of using a selection of different gradients and setting them to the relevant blend mode. There’s a breakdown of each blend mode after the jump.