I think it is a matter of preference and style. I consider myself first a sculptor then a painter, because the substance inside my mind is literally like clay which I can shape into any form. This is why I work from dark to light (for my 4 current paintings); I'm sculpting in 2D rather than thinking about painting a picture, and color is is secondary to form. The entire painting this way is an additive process. It's not without it's flaws though... it takes a lot of building to eliminate the dulling effect of the base layer for example. But imagine trying to generate some swirls in Apophysis or some other program, then paint that on a light background, trying to blend all those fine wisps and gradients of color mixed gradually into black paint on a white canvas...oh it would be painful. I wanted these paintings to have that level of detail and appear to be built of light rather than shaded the usual way.
When working over a mid tone you can build up and knock back from the start. But you are building up from the middle (this is how I think of it). Shadows must be added, instead of leaving the negative base layer as the shadow itself.
Working over white is all about killing the light and getting it down to the mid tones, then chipping away with shadows and adding light. I think this is what most people do and it's the way I was taught.
MelCat, love it, the new perspective on it really adds depth too.
