Normal maps for simply textures

I have a rather noobish question about making normal maps.
So far I’ve been using CrazyBump to make normal maps, but in that case I have to use photos of real life textures.
How do I make normal maps for textures that are merely a color though? Made in Paint for example.

I use Blender to model and the models I make are for Gmod at the end (in case that’s useful information).

Why not model in the details and then bake using XNormal or Blender’s built in?