Never done a comic but when I was doing this map, I used textures to make it look more interesting.

Spoiler:


Even the sky is a texture/seamless background thing XD

So basically I coloured it all and then took a texture like these (make sure it's seamless if you have a big space to fill) and made it a pattern on photoshop and then filled the layer above where I coloured it and set it to 30% opacity or something. Not sure if that's helpful but I found it makes big blank-ish spaces like in backgrounds more interesting than just plain colour. xD

@Suicune's Fire