Toning the canvas and values for white.

Its a good idea to tone the canvas before starting to paint for several reasons.  First of all  it helps you to judge your dark tones as it is hard to see any difference between your darks if they are placed on white.  Secondly it can be use a eiither the color cast of the scene caused by the light or as a complimentary color to vibrate and make the image pop.  Lastly i would recommend toning the canvas in a tone that exactly matches the color of a white object in the light, if there are any spectacular highlights or light sources in the scene.  this makes sure that you always have enough room to push up towards the brighter tones of the highlights.