Changing the color of a custom skin is quite easy, on the picture below you see a color palette and a little cutout from FOEdit, the color palette is where the game gets the color for the remapping index, its only for reference.

To change the color of the custom skin open FOEdit and search the material swap entry of the custom skin, then you need to change the color remapping index (0.09 on the picture), changing that value basically moves the color picker up and down, higher values ​​are down, lower values ​​are up, the FOEdit example on the picture is black which has a very low number, 0.5 would be yellow etc. I recommend to look at the values ​​of other material swaps from my mods.