I know this isn't possible with the LG software, but wouldn't it make sense to allow us to change text message LEDs universally, rather than by each contact? Can we assume this will get updated in a software release?
I don't mess with the LED notifications. The reality is that if I see a notification, I'm going to pick up my phone to look at it regardless of who it came from or it's format.
But, I know many folks use Light Flow from the play store. I'm not sure whether it has the fidelity you are looking for, but it might be worth checking out if you don't already use it.