Adding texture is a great way to bring in more colours. I place a number of rust textures over the painting and lock them to my three base layers with clipping mask. On the left side is how it looks like when you put those textures on Lighten mode. I love this layer mode because it will only affect those colours that are darker than the texture.
Giving my character more hair
To be honest I’m really lazy when it comes to painting hair, so I use a selection of one of my older paintings of a girl who has wild-looking hair. I spend time altering it with the Transform tool so that no one will recognise the original source material. I think I’ve pulled it off! All the single hair strands also instantly make the image look more detailed.