One of the easiest ways I know to create a coloured sketch quickly is to rough it out in pencil, lightly add fairly fine pen work ( using a fineliner), add a few scribbles of oil pastel for colur and texture where it would be useful, and complete your work with watercolour washes.
The oil pastel created fantastic texture for wood, stone etc and the fineliner work gives some definition and adds some darks. The watercolour washes add colour – remember to leave white paper where you want highlights.
This example is from my current series of sketches from my recent trip to India. 
