Facial hair (and head hair) of any kind never gets in the way of combat. A new tissue layer with the same properties should be fine, so it can say "her tattoos are blue" etc.
There are also "shapings" for tissue layers which at the moment apply only to hairy bits. The [TISSUE_SHAPE:STRANDS] is what makes them irrelevant to combat. The TISSUE_TEMPLATE for each of those has [COSMETIC] and [STYLEABLE] tags, which apparently allows them to use the shapings in the creature or entity file.
The problem is, the only shapings available are the hard-coded NEATLY_COMBED, BRAIDED, DOUBLE_BRAIDS, PONY_TAILS, CLEAN_SHAVEN, STANDARD_HAIR_SHAPINGS, STANDARD_BEARD_SHAPINGS, STANDARD_MOUSTACHE_SHAPINGS, and STANDARD_SIDEBURNS_SHAPINGS. I don't know if anyone has tried to make a shapings_custom.txt file or what its syntax might be.
Getting that syntax, if it's even possible, would probably require talking to one of the gurus who look at dissassemblies and string dumps. If it's possible, you could have the color and the "shaping" be independent of one another. It would appear as His/Her (color) tattoos are (shaping).
Her black tattoos are in a geometric pattern.
His crimson tattoos are depictions of weapons.
The much-less-appealing alternative would be to use a bodypart appearance modifier (look for BP_APPEARANCE_MODIFIER tags in CREATURE files). The choices are hard-coded, and in my opinion none of them are generic enough to apply to tattoos. That is, there's nothing along the lines of "very small" to "very large" (If I were to guess, it's to discourage modding in boobies and stuff) or "simple" to "elaborate". The BROADNESS range might work for describing war paint stripes.