The mod "ImmediatelyFast" makes a custom crosshair color just not exist (and just sets it to white). I had quite a few other mods, but it was just this one mod that had produced an optimal change when I disabled it.

Its Modrinth page had stated that it does affect the GUI/HUD, so that's probably why, but it's best to put that in a list of incompatible mods (or attempt to fix it, I don't know lol) so others don't have to test every mod like I had to. I'll also put this in their issue tracker so it could be fixed on their end, if possible.

Another thing to note, custom colors WILL work if "Use Dynamic Crosshair Style" is set to OFF. I don't understand why this is, but it works. I just can't have it change crosshairs anymore.

This is because ImmediatelyFast does some caching/batching for unchanging elements. Well, the crosshair is no longer unchanging, so it's incompatible with DC. I can add a note.

Closing this because, starting with their current rewrites in 1.21.9+, Mojang adds most of what ImmediatelyFast offers to the base game so it won't be needed anymore anyway.