I think it's all about the situation, the person, etc. I'd err on the side of being generous to a certain extent.
My first experience of someone asking for paint was very similar to the OP's story. A teenage kid with a loaner came up asking for hand-outs. I was a little annoyed at first and didn't giving him any because I was down to my last bag myself, but thinking back, I wish I had. I don't think he was being especially rude. It was just his first time and he didn't know how much he'd shoot, what to expect, etc. So he didn't bring enough money. And since he was a teenager who probably lives off his allowance and whatever his parents give him, his default mode was to ask for what he needed. I mean, if he's going to a movie Friday night, he probably asks his parents for spending money. So if he runs out of paint, he's going to fall back into that habit. Maybe not the best habit in the long term, but he's just a kid. He'll grow out of it.