First I open the BB skin for the race the face shall have (most of the times it's orc for me
) to have a direct comparison while adjusting the skin tone in irfan. then i load this newborn facetex into paint. after i got this i open the original texture also in paint, cut out the eyes and the teeth to get back the white. if i wish, i adjust the iris and the lips in a way i want (to have two coloured eyes and darker lips - to do this i often alter existing textures of lips and eyes from ren and westly, just cutting them out and pasting them in - of course none of these texes will ever be published). save. have a look at it in nifskope. if required i adjust the tex so it fits an orcmesh and save again.
still in paint i cut out everything except the iris again and make the whole background black. this is my glow then. i adust the colour of the iris to my liking in irfan. have a look at it in nifskope. be sure not to overwrite your first file by just hitting the save-button.