Little Cheep- Grisaille Image

This is the black and white image I will use to paint from in order to establish my underpainting.  I did not take this picture myself but obtained it from a photo library on Wet Canvas.  I bumped up the contrast a bit and lightened the overall picture.  I want the underpainting (the black and white stage) to be slightly lighter than the finished picture.  When I begin to glaze colors over the underpainting, the overall picture tends to darken somewhat.  My goal is to tackle problems in the early stages.