Yep, I'm bringing this topic back. And I'm kicking it off with the start of a much-requested tutorial!
It's not the most detailed of tutorials (I have a bigger one planned), but it should help. For the tutorial, I decided to colour one of Borock's images. Here it is so far with the skintones coloured.

How did I get to that point, you ask? I'll tell you how! Well, I'll at least give you a general idea how as it would be too tedious to go through each and every brush stroke, and a good bit of it was done by futzing around until it looked right. Futzing is something that can't be taught. Anywho...

I opened up the image, cleaned the linework a bit, and then made the background into a layer by Right-clicking on it in the Layers pallet and selecting "Layer From Background." Then, I changed the Blending Modes (indicated in the image) to Multiply. After that, I made another layer and put it below my linework layer.
That layer became my shading layer. I decided on a lightsource and shaded it in grey. I changed the Blending Mode of that layer to Multiply. Then I made another layer for the colours.

Hide the shading layer. Then put your base colour on your colour layer. Labeling your layers may help you keep track of 'em when they get numerous. I usually don't label mine as I just keep track of what's on what layer by hiding and unhiding it and seeing what colours disappear.
Once you got your base skin colour in, make a new layer. This will be the first level of your skintones.

Put each shade on its own layer. If you are unfamiliar with facial shading, references may help. I usually have at least one or two references up just to make sure I'm getting the shadows right. Usually two-tone anime and cartoon shading will not require this level of facial shading. It's more of a personal preference, so shade however your style dictates.

Once you're done with the facial shading, unhide your cast shadow layer. It will prolly end up being too dark for your liking. There are several ways to fix this. You can either go through the blending modes to see which one works best or adjust the opacity. For this one, I just adjusted the opacity down to around 40%.
Now onto the arms...

For the skintones on the arms, put each shade on its own (or corresponding, as I did) layer. If you are unfamiliar with anatomy, dig out some references to help you.

This is another personal preference, but I felt that my skintones were too harsh, so I used the Soften Brush set at 30 % and blended the layers. Then I unhid the cast shadow layer and saved my progress as I was starting to get tired and didn't wanna end up rushing the colouring. After unhiding the cast shadow layer, I felt that it looked a bit flat, so tomorrow I plan to add to that layer after I colour in the shirt, hair, paddle and such.
But there you go, a basic jist on colouring skintones. I'm still perfecting my technique, so this definetly will not be a tutorial to end all tutorials.