Checking the values of artwork as you work allows you to disregard color for a moment and evaluate the pattern of light and dark. Transforming the view of an image into black and white is very easy in Photoshop, using a Black and White adjustment layer.
Many thanks to David Opie (www.spacemandave.com) for this simple but very useful tip.
Select the topmost layer in your document, then create a new layer thusly: go to Layer> New Adjustment Layer> Black and White and poof, the entire image is in grays (though none of the layers below are actually changed.) How does it look now... flat or contrasty, too much dark in one area, too uniform, or whatever? In the case of this Allosaurus on the left, he’s as flat as a pancake and needs some shading and highlighting.
That’s better, isn’t it? The way I’ve been doing the modeling lately is to make one layer for highlights set in Screen mode, then a layer for shading set in Multiply mode, both clipped to the color layer. The same color can usually be used on both layers but because of the different modes, it has a completely different effect. I‘m still not happy with his teeth because they don’t look sharp enough, but that’s a problem for another day!
Here he is with the Black and White adjustment layer in place. From now on, I’m going to put a B + W layer on top of every image and use it to punch up my artwork. You can turn it off and on by clicking on the eyeball.
I’m still slogging away at these prehistoric illustrations... hope to finish before the next asteroid strikes!