From what I've noticed SRP skin shading has a lot of,, weird hue choices, but in color theory terms I've noticed it's like this
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The more darker the closer it is to blue/purple on the color wheel, the more lighter the more closer it is to yellow on the wheel. Darker is more desaturated, lighter is more saturated. Usually the eyes have a drastic difference in hue/saturation/contrast changes in color to appear more IN-DEPTH.
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As for placement on the skin of these colors, I'm unsure, that's debatable per person to where you just go nuts with it. Also another thing I've noticed is that the color differences are /very minimal/ so shading isn't THAT drastic in most people's skins on the server. Also most skins are very de-saturated and have a yellow-undertone over all the colors.
I'd recommend using a site like
planetminecraft for finding bases labelled under
FTU to mess around with and using a program like novaskin, minersneedcoolshoes or blockbench to edit skins. Blockbench is the easiest to use if you have art-program experience and has the most features but the others aren't bad either.