dyeing pbt works best at below boiling temp and you control the saturation of the dye by the length of time you submerge them in the dye bath + the amount of dye pigment in your mixture. dyeing pbt should not have any negative effects on the legend crispness, but there are some other downsides in my experience:
dyeing colors is multiplicative, so it works best with light pastel-ish colors on TRUE white keycaps (otherwise you will need a little more advanced color correction knowledge to neutralize the "cream" colored keycaps that are the most common in BOW sets). as an example, if you were to dye EPBT Kuroshiro "BOW" keycaps with a royal blue dye, you would end up with a slightly muddy greenish color because Kuroshiro is not white. it is a very light reddish-yellow, as is the case with most BOW keycaps. you would need to neutralize the base color with a small amount of purple mixed in with the blue for it to net a blue color in the end. also you can only go darker not lighter.
multiple dye baths, or very prolonged submersion, or overly high temperature can cause discoloration marks on the keycap surface (these look like discolored splotches unevenly dyed compared to the rest of the cap). going for a very bright saturated dye color or dyeing a light keycap to very dark will require extended amount of dyeing and will likely subject to this discoloration effect.
not all pbt is the same. some "pbt" caps are not all pbt (for example PBTFans keycap base is pbt but the doubleshot legend is not and the legend will not take dye). different blends of pbt will react differently to dyes
some tips:
when setting up the dye bath, i recommend adding in a small amount of clear dish soap + salt to the mixture. also dont put in too much dye pigment into the mixture
always dye completely clean or brand new keycaps
always do a test run on spare keys you wont use (and im talking about from the same set you plan to dye not a random set) to get a feel for how the dye reacts to that particular plastic
never trust the advertised color of the dye, always test like mentioned above. you will very likely need a blend of multiple colors to achieve what you want.
dont have high expectations for bright or neon color dyes, and likewise do not have high expectations for changing a light keycap into a very dark one.
use a plastics dye that is specifically made for synthetic polymer based plastic.
and lastly remember, a dye bath will alter the entirety of the keycaps, base color AND legend color (if we are talking about using dye-sub pbt)
