I have painted the alien minion in order to decide which aspect will have and which palette I will use in the final model.
I have isolated the painting surface with alpha masks to delimit the painting surface. I used also a multiply layer filter on the drawings to keep the lines and be able to paint under them. After that I created a base layer with 100% opacity to support the upper layers transparency, I painted the base colour of each part. After that I started the lighting process by adding the gradient colours with a smooth brush with high opacity, one layer for the shadows and another for the light. In order to keep make more contras in the silhouette and some interior lines I repainted some of the lines with black and then to increase the lighting effect I added a second line of white next to the black line in the side the light was coming. Once I finished I started using filters and modify the colours to get the desired effect of the base colour model. Then I started to change the hue, saturation and illumination to test and get different colour results helped by layer mask filters in order to get some colours that are not obtainable with this process, like white or black.