I think @creativetruth, you ask a very good question. for a good choice of colour there are a few theories that we can fall back to. For me, I tr to understand the basic of colouring. When I choose my colour, I will take the medium range of colours. This is to make my job in controlling the temperature of the colour on the pigment. To increase the pigment colour I will use overlay and this is to increase the speed of the entire process. I do a quick post mortem on my earlier painting:-

1. To get the medium range of colour I use those within the range of the red line in pic 1.
2. Then I reduce the opacity to 20% to obtain a balance to the blue colour. You can use the reference in pic 1.
3. You can start to blend the colour by using a brush on a normal layer with 100% opacity. -
4. It is important to increase the pigment colour. I will use the overlay to increase the temperature of the colour.
5. For the shadow, I use the darkblue with opacity set at 70%.