I think I have this same issue with red/pink and when a color is one or the other in that in-between area. I also do this with blue and green as well, though I feel less so than pink/red. I also have done colorblindness tests myself and do not test as colorblind (either variety).
What percentage of Signal users is “grandma” that uses Linux and would be messaging from her PC? I would have to imagine the overwhelming vast majority of Signal users are on mobile only, so packaging for specific distros is probably far down the priority list.