I used a pencil tool with a base colour set to 180 opacity. This base colour I covered the entire head in. From there I went on about optimising each colour in this layer, so that it would amplify the shading and colouring of the base layer, instead of just greying it out. To adjust saturation I used a few post-editing tools, primarily just a basic saturation changer in paint.NET, which I am sure you can find in other equally good or better softwares. To make sure I didn't run into bad edits, I manually selected which colours I wanted an amplified saturation for. If this did not turn out well, I manually changed the colour (still at 180 opacity) to make sure it fit.

The manual work was done in MCSkin3D, but could just as easily have been done in paint.NET entirely. I hope this helps.

I am praying it won't take several years. But maybe I'll forget to ever try again q:

Thank you! Also quite surprised how some of the colours worked out.