The customer is actually asking the shop assistant for their opinion, not just asking a rhetorical question.
In other words:
I'll take this green sweater. I like the color on me. Do you like the color on me too?
I'll take this green sweater. I like the color on me. What do you think?
In this case, asking somebody else I like the color on me, don't I? wouldn't make sense. It's not an obvious enough situation for the question to be rhetorical, and a questioner doesn't normally ask somebody else about the questioner's own feelings.
If it had been a rhetorical question, they would have stated something obvious and then asked a question only to emphasize the situation rather than to actually look for an answer.
For example, after eating three servings of a meal:
It seems I was hungry, doesn't it?
Or to somebody who's finished watching a particular movie for the tenth time:
You really like that movie, don't you?
Because the answer is self-evident, the questioner isn't actually expecting one.
Now, if the shopper came in every day for weeks and always bought a green sweater, then a rhetorical I like the color on me, don't I? could be appropriate. But there is no context for that in the simple sentence given.