Shattered Kingdoms

X exclaims to Y 'Go away! Gain some more experience before coming back here!'

That would be fine for some people to say, but others it doesn't really fit the character. Could there be a list of options that can be used to replace this within the game? It would be even better if we could come up with our own messages, but that could get messy to have to try to police.

Examples:
X says to Y 'It seems that you are not quite experienced to learn that. Why don't you travel a bit and explore?'
X exclaims to Y 'You lack experience in the world. You should go out and see what is there!'

What about:

I like it. Even simply just changing the default response to one of those two would be an improvement.

Make it by aura.

Darkies be like "You are wasting my time, get out of my sight and go train."
Greybies be like, well, they could say a lot, depends on the greybie
Lighties be like "I would give you some learning if I could."

X tries to give Y some instructions but Y doesn't seem to understand.
X tries to give you some instructions but you can't understand. X returns your coins.

I favor this one. It nicely handles the situation for anyone in question and doesn't force words on someone's character.

Not entirely newbie friendly and intuitive. "Doesn't seem to understand" doesn't really state clearly it's due to lack of experience.

Perhaps something along the lines of "X tries to give Y some instructions but Y doesn't seem to be capable of learning something at this time. Perhaps train more?"

I don't think the standard line has to be newbie-friendly. That's where the oldbie, the mentor, steps in and puts his fingers on the keyboard. The oldbie sees that line show up, and takes it as an opportunity to engage in in-character conversation as a result. I understand your position. You desire for any "new and improved" message to be as informative as the "old and worn out" message it is replacing. But I don't think it is necessary, as I have (somewhat snarkily) suggested above.

Your suggestion will work the way you intend it to work, and is acceptable as such. My suggestion would allow for a more personal touch, a more in-character response, more involvement from the mentor.

Keep in mind that it's the same message from a PC mentor and an NPC trainer.