Now that the pencil drawing is complete I kind of go backwards...I erase each area where I want just white and draw in the white areas. White over 3B pencil just doesn't work in colored pencils, at least not for me! Then I find the next lightest color, which is a light blue and start adding a bit of that to the lightest areas.
Sorry for the horrible images. I thought my camera phone would do better than it did!