Art?

I'll typically start with basic skin tones for the character (above) and on two separate layers begin to paint on some variations of red, orange and/or yellow (layer 1) and blue, teal, and/or purple (layer 2). I typically already have the opacity on these layers down, but just to kinda show you the types of colours I may use and where I place them, this is the kind of monstrosity I create:

In my experience, Yellow is a good neutral zone addition, Red is great for places with a lot of blood flow like cheeks or fingers, and Blue is good for areas that may not get full exposer to the light source(s). I was first taught a real simple gradient down the face of yellow (brows), red (cheeks/nose), and blue (chin, jawline) for faces. Then as you get more comfortable with slapping them on the canvas, go a little wild with it and start intermingling the colours like adding blue on the eye lids or red on the lips, for example.

For Layer 1, or our warm red/orange/yellow colours, I typically set the layer style to something like a Multiply and lower the opacity to 15-30% depending on the skin tones and image's lighting.

For Layer 2, or our cool blue/teal/purple colours, I do a highlight layer style with Overlay typically being a nice safe pick. For this layer I generally stay around 20-45% opacity. After you have that all set, you'll get something like this:

Which is thankfully not quite as jarring as the other version haha. From here, I start shading normally! I typically use 2-5 shading and highlight layers, depending completely on the lighting of the piece. For skin specifically, I try to keep a smooth gradient or matte look to these areas with a soft brush and blending. Back lighting and colours from the surroundings generally seem to bounce off skin well so I tend to utilise a lot of that too.

In the end I wind up with the top(which I now realise was a pretty bad example image to use as they are in a SPOT LIGHT I'm so sorry) and also added the version without the extra colours for comparison(bottom):

I know it's nothing SUPER noticeable in this piece if you're not looking for it, but that's also the point! It's a little bit of flavour without being a distraction. I've personally really enjoyed the overall look of the skin I've painted since incorporating this.

On the other hand, when it comes my single layer paintings (using my recent painting of Beanix as an example,) I couldn't really tell you my process besides throwing colours at the canvas and seeing what sticks. I still try to incorporate the "sway my colour towards yellow here, change the hue to a bit more blue there," but there isn't much method to my madness outside of that. Just have fun with it!