I love starting my sketches with a watercolour wash rather than ink lines. I find that when it is just me and the pigment floating on the paper, I capture the light better and get more interesting coloured washes.
I think this is because I am purely focusing on the coloured shapes and therefore see the colour better. For some reason having ink lines on the page can affect the way I add colour – I am more likely to follow a set sequence of steps, and play safe with colour choices. When the paint goes down first, the paint has much more of a say about where my sketch is heading and I have been known to drop in some completely random as well. I love giving up a degree of control to watercolour pigment.
There are some bleeding issues associated with drawing over the paint (whether it is wet or dry), and it often isn’t as accurate as I can achieve with lines first, but the trade off is worth it – for me anyway.