Gouache

To achieve a layered effect, I experimented with gouache paint, which can be used in a very diluted form (like watercolour) or in a thicker format (such as poster colour). 

I can also build up the colours by applying layers on top of each other. For the lighter areas, I diluted the colours a lot so as not to let it dominate too much. The darker areas are much thicker, allowing me to put more than one layer on certain areas.

I have to keep in mind to not overdo the layering, as the paint would eventually crack when it becomes too thick.