@Random_Passer-by
Yeah, it would require an ink change/different technique/equipment/something along those lines.
It would increase the cost/time on physical copies, and while not by a huge amount individually, given all the copies it would certainly add up.
Given current commonly used equipment/techniques there are some ways they could work it without too much of an increase, but it'd likely require time/expense on planning the changes and the like since it's not something done (much, if at all) atm.
It could much more easily be done in the digital copies, and they could release a limited edition (other additions too of course) with an increased price too, though idk that the success of this is enough to warrant that yet. The digital copies is the real way I would expect it, especially now that there is a growing number of series to be released digitally first (some only digital as well) nowadays.
Anyway, more details and discussions possible, but this is already more than probably needed lol. Even given all this I want dem colored eyes. My unreasonable demands must be fulfilled!