Hallow has pink water, and Jungle has green sky. It's not realistic, but it's atmospheric.
It's just as confusing as the different colored waters. "Is this safe to touch?" You test it and then remember that it is or isn't.
Water can turn yellow, red, pink, and purple.. I'm just not seeing why it's such a big issue when it happens to lava.
@jordan has already shown that it's possible to not only change the color of lava, but inflict debuffs as well. The only apparent problem is the red glow lava emits, however that doesn't sound impossible, just takes a little more work. It would definitely take less work than rewriting the whole Terraria engine to allow more than 4 liquids for liquids that have identical effects. And then you'll have normal lava in corruption that spread over. It's way easier to just have the colors change. They didn't make new liquids per water color type, so they don't have to with lava either.