Sam Nielson: Another painting process-adding color to grayscale

I know a lot of people who like to paint their values first in black and white and then apply color to their values.  I've never found an ideal way of doing this, but I've learned a couple tips that help it work out a little better.
First off, I don't let any of the values get too dark, except where I want things to drop completely into shadow (ie. black)  In this case, I lightened the "grayscale" image before applying any color to it.  You can see the original values I painted on my blog.  I usually don't spend a ton of time working detail into the values because I often have to paint in all that detail again when I get to the colors.

Then, I use a multiply or gel layer to control values.  The "Color" or "Colorize" layer types often result in values different than what I intended.  Painting a "Gel" or "Multiply" layer with bright colors will gives a similar result, but I like the option of bringing the values down where things aren't working out.  This is what the layer looks like when it's over a white canvas (or a white layer between the canvas and the color layer, that way I can just change visibility if I need to pick colors off the canvas).  Notice that I put in color changes from light to shadow; otherwise, the image looks like you've colorized a grayscale image because all the light has a uniform color.

This is what the image looks like with the colors applied over the values.  I usually only take this coloring step so far---it's useful for blocking in big colors and even playing with the values a little, but once it's looking pretty good I collapse the image so I don't have to wrestle with the layers anymore.

Now that the image is flattened, I use a "Screen" layer to put in light reflections and a "Lighten" layer to add some sub-surface scattering.  Then I collapse again and use a small brush to paint in all the details.

So when should you use this method?  Sometimes it's easier to handle the values separately from the colors, especially when your lighting is fairly simple but you've got a difficult composition.  Another time it's useful is if you want patterns or value changes across a surface without messing up the values (painting these things across a lit surface can be a pain to figure out).

The reason I don't often use it is the pain of dealing with the value shifts when colorizing.  If I already spent a lot of time working out the values, I don't want to work all that out again.  If the values are very detailed I sometimes feel like I'm painting the whole thing twice.  However, this process suits some people's patience and way of thinking so hopefully it's worth sharing anyway.