Speaking from experience, in order to "cover" an old tattoo, it is always best to go with darker colors over lighter colors for a couple of reasons-
Light colors always fade first. White pigments dont cover anything, even skin tone, very well to begin with, and they can vanish from the tattoo in months.
You are basically inserting the new pigment into the same space the old pigment occupies, and you are essentially reopening the original wound, exposing the old pigment. The colors don't really blend, say white pigment over red making pink; instead, you get mottled red/white/hollow. Blue over red gives you really dark, blotchy blue. This happens because the lighter the color, the smaller the pigment. The smaller the pigment, the easier it is for the systems within the skin to break them up and subsume them, and these systems always go for the easiest targets first. (This is one of the reasons why you use salves on tattoos while they heal, and why tattoo artists push for the use of natural salves like A+D ointment or lanolin based lotions versus Vaseline or bacitracin- in addition to keeping the tattoo moisturized to speed healing, it also gives your skin something to attack other than the pigment, giving the pigment a better chance to reside in the skin until it is sealed in by healed skin.) In any case, some of the old pigment goes, some of the new pigment goes, and in spots all of the pigment goes. I've actually seen an artist load up white pigment into a tattoo in which the black shading "bled" to blue haze, and when it healed, the haze was gone, and the white went with it.
Also, when you are doing the original tattoo, the process that you use to color is actually the reverse of what you would use for coloring an illustration on paper; tattooists go dark to light. Dark colors go in first, then light, then lightest, then pigmented highlights. The reason is because the darker colors show through the lighter colors, so shading in colors can be accomplished in the skin in initial tattoos. However, on "coverups" what you get is fuzzy, broken black or bluish lines showing through the color where you've gone over initial line work or black shading. When covering, you try to integrate the original line work and shading into black areas or the darkest colors of the new tattoo.
Incedentally, Wrecking Balm does not remove all the pigment from your skin, and lasers do, but leave hard to observe scarring in the skin that precludes successful healing of a new tattoo in the area. Scar tissue also makes tattoos hurt MUCH worse.
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