Being an old monochrome pen-&-ink and graphite type, I have a pronounced tendency to fall into full tonal black and white renderings, unreasonably early in my drawing process.
However, when the topic is REDheads, this'll need a little more work.
All right then, let's drop in some more-or-less flat base colors . . .
. . . and use the old Multiply mode trick with the BW layer:
Uhm, eeuuuw! Well, that's gone a bit necro, hasn't it?
[Side Note-- This is one of my pet peeves with the default settings on consumer-grade 3d rendering gear: all the shading being added via deadening grays. On flesh tones, no less. What could they possibly be thinking?]
So, in years past, I used to use Image->Adjust->Curves with a nice preset I saved, to convert my grayscale layer to sepia. Which at least got the fleshtones to behave. But, it made touchup edits awkward, the layer no longer being in simple gray. And other items on that layer didn't necessarily need to be shaded in skintone sepia either.
Well, on an as-yet-unreleased project, I stumbled upon a sort of backwards use of the Overlay mode. Traditionally, you use it on a grayscale-patterned layer, to apply texture to a color layer below. But you can also use it to apply color tones to grayscale. So, here's Sylvia's overlay layer . . .
. . . and its effect on the BW layer:
Used a traditional orange-brown sepia on the props, and a magenta range for her skin shading. Neutral on the sclera and blue for the irises. And yet-- I can continue to edit my still-monochrome BW layer, while immediately seeing the final effect on the color image! Sweet!
Finally, flatten all the layers together, et voilà!