A dark shade of green could be the best compromise, as green is one of the color we have the easiest time seeing variants of (sorry color blind people).
Red, on the other hand, is the color our eyes notice the most, so any shade of it would make for a poor OOC color, as the goal for that new color is to be less noticeable than the classic white/yellow for the usual Say and DM Texts.
As for Blue, it would either be too bright to properly serve its purpose or too dark to not be considered Grey and thus be "a pain in the ass" to read, as it's been said previously when the OOC text was a dark grey color.
I personally have no issue with the current purple being hard to read (depending where you're at, though). As I - and others - said : it's supposed to be easily ignored. Then again, it shouldn't be a shore to read, especially when OOC text is used to tell people when you're going AFK for a bit or need to logout for IRL reasons or whatever.