I use bright pink (R255,G0,B255) for tranparent pixels so I have to put
This is a little off topic, and not really relevant, but #FF00FF is
(anti-green; the primary pigment color commonly mislabeled as red in
children’s books), not bright pink. Pinks are mixes in which green and
blue are equal to each other and less than the red.
I like to reserve R,G,&B values of 0 as having special meanings because
it is easy to test for, easy to avoid, and values of 1 are visually
indistinguishable from values of 0. Maximum color values are also good
choices for the same reasons. Accordingly, I think maximum magenta is
quite workable (if a little more complex to avoid than #000000), though
I wouldn’t recommend using a true bright pink (say #FFAAAA) because if
you ever decide to use mathematical color modifications (fades, filters,
smooth scaling, et c.), you might bump into it by chance.