This post did turn a bit but I'm glad people are exchanging ideas at least.
I have a question for Mr. K though. I agree that maybe 'Don't be scared' and 'You lie' are pretty rude and there are more subtle ways to take a compliment. You mentioned 唔係呀話 which I never hear that much, how is it used different than 唔係呀嗎 （I hear this other phrase a lot)。
As far as if it's a good idea to use complicated 成語 after a compliment I really don't think my input means that much. Donal I think your Chinese is better than mine and you have to deal with more of these Cantonese interactions, so I don't want to disagree. But my own personal opinion is that all a fluent foreign / white speaker needs to say is a simple thank you and then use his language skills to separate himself from a beginner (The speaker would use good tones, competent grammar, slang, and show a knowledge of Cantonese culture / cities).
Using English as an example (because my current Cantonese level in speaking or writing can't convey these thoughts that well).
Person A: Hey, your English is getting better compared to last year.
Person B: Thanks, I've been studying a lot ( .... and no need to say anything else, just the two people will keep talking in English)
2nd conversation, all in Cantonese:
Person A: Your Cantonese is really good.
Person B: Thank you. I still have an accent I think. Anyway, were you meeting us to watch the basketball game with so and so? (... and everyone continues to speak in Cantonese).