I often receive cries of help when people cannot adjust their prim attachment matching the skin tone. To understand the colour menu on the HUD coming with the item or in the building menu helps a lot.
When on a white texture, I usually preselect the about-right colour clicking right into the colour field. A good start is the “Show RGB under curser” in the advanced >> menu. Then move it around until I have the right tone and saturation, after that I adjust if lighter or darker.
This is usually easier as the colour you define is what you get on the prim, when working on a non white texture, you have to find the right colour to add to the texture in order to have a match. You can only add, means if the lightest skin-tone on the HUD is to dark, you cannot make it brighter. In most cases you work with really desaturated neutral colours here that you add to a skin tone.
When using a HUD I start with the closest skin-tone* of the preselection which needs to be lighter and possibly less saturated (grayer) than the skin. And now there is a lil bit of science to it 🙂 when the tone on the prim is too red, add a greenish (complementary color) desaturated gray, if too yellow add purple and so on. Too bland? Use a more saturated colour. Too vivid, decrease the saturation.
After adjusting the tone, finetune the lightness and if necessary go back to a fine tune of saturation and tone.
Practice makes the master – Good luck
*in a few cases the preset skin-tones are way too saturated or dark, no use trying here, devil the creator of your prim attachment to supply a lighter tone.