Before we get into the meat of the tutorial, I'm gonna show you the differences between using the same and different colors for both shading and adding highlights.
For the left circle, the Hue, Value and Saturation wasn't really moved when picking colors, this in turn makes the colors seem "dirty". Skin will always have some kind of tint to it.
As for the right? For every shade i picked, the hue was shifted slightly as well as value and saturation.
This can make a difference in your art, which can help convey the mood you're trying to go for more accurately.
1. Dense / Opaque watercolor
2. Blur / Blend
3. Soft / hard airbrush
All these brushes are in CSP by default. The settings for them are all default as well.
Start by eye dropping your base color and pick your shading colors accordingly. Remember to move the Hue, Value and Saturation slightly with each color. You can add more colors to your palette but I sufficed with 4, including the base color.
Create a new layer above your skin layer, leave blending mode to NORMAL.
Make a gradient using the SOFT airbrush.
I like to add this first color around the cheeks, hands and neck. I like to think of this step similar to that of applying a light wash on a canvas before starting painting. Instead of starting with a pure color for the base, we have a light tint in surrounding areas of the skin.
2. Subtle Shading
Make a new layer, set this one to Multiply. Make sure its above the previous layer.
Using the DENSE watercolor brush, we're gonna add some subtle shading. For this, I like to add it in places where I know a shadow will always be, for ex. on the eyelids, under the chin, nose. I will also add it in places where i think best suit the drawing.
If i need to erase, I'll use the TRANSPARENCY instead of a regular eraser. This way, the dense watercolor brush will act as an eraser. Using the transparency can also double as blending tool to create soft gradients.
With a BLUR too, I'll create some soft edges to make the shading seem seamless. You can also do this with the BLEND tool.
I'll do this on the eyelids, and on the neck primarily. Try to keep a good balance between soft and hard edges.
3. Actual Shading
Create a new layer above your previous layer, leave blending mode to NORMAL.
At this point your layers should be as follows:
BASE > NORMAL (GRADIENT) > MULTIPLY (SUBTLE) > NORMAL (ACTUAL)
Use the last 2 colors interchangeably to define the shading you want to make more prominent in your drawing. Blend/Blur in areas you wish to look seamless. Adjust when needed.
I like to keep hard edges within the ear, neck to show muscles/veins and around the cheek area. This can be adjusted to suit your preference.
This is what my final piece looks like.
3.1 Adjustment (optional)
This step is optional but you can use a layer set to OVERLAY to adjust your colors using a SOFT airbrush. Make sure to put this layer above all your previous ones. I'll use various shades contrasting colors to either mute certain colors, or complementary to bring them out.
By going to LAYER > NEW CORRECTION LAYER, you're greeted by a ton of options for color correcting, my favorite is the TONE CURVE.
You can play with different blending modes and opacity levels for this step.
Optional: Add a layer set to MULTIPLY above all your colors, I think this helps bring out the lighting more but again, this can be omitted if deemed unnecessary to you. This layer can be filled with any color you think best fits the atmosphere/mood of your drawing, in my case, I'll be using a light beige. Play with the opacity if needed.
Create a layer above all your colors (above the multiply one as well) and set it to SCREEN. This is what we'll be using for the lighting.
Eyedrop your base color, this will be the highlight color. Try to stay away from using pure white.
Where you add your lighting depends on where the light source is. Color and blend accordingly, you cause any brush for this.
You can saturated orange to the edge of your lighting to create a subscattering effect. Feel free to also add any more shading you feel is necessary with a MULTIPLY layer and adjust the colors as desired.
And here's the end result!!
Below is an example of one own finished pieces! Happy coloring ~