Produce all your work in greyscale, concentrating on the greyscale values and contrasts to get the right mix. I am using ArtRage for shading and then whacking it into photoshop for colouring at the moment. Use any brush in photoshop and set it to color mode and just simply paint the colour on the painting.
Don't forget to set lock transparency on the layer for easier painting. This method wont produce mind blowing colour work like doing it normally, but it is fast and efficient.