Well you can always try the "friendly but stupid" approach LOL. Ask them something like "gee those are pretty snakes, what makes them a pastel?" (basically in a tone indicating you've never seen a pastel in all your born life LOL). If they show you how these normals are pastels just smile nicely and leave. Come back the next day with pictures of actual pastels and again plastering the smile on your face say "gee I did some research and these are pictures of pastels from breeders, how come yours don't look like these?"

Hopefully they'll get the point LOL.