Is it correct and totally colloquial to ask someone "What are your favourite foods and drinks?" in order to ask about what different foods and drinks they like, not meaning the dishes? or should one use those words in the singular?
There's nothing wrong with the sentence you posted (although it sounds a little "heavy" to say), but to be totally colloquial, depending on the context of course, I'd go with something like:
What kind of food and drink do you like?
When asking someone's favourite, I feel that food and drink would rarely be asked about together, but separately as:
What's your favourite food?
What are your favourite foods?
What's your favourite drink?
What are your favourite drinks?
If you were interested in them both together, and how they compliment one another, it would probably be asked as:
What's your favourite food and drink combination?
Or you could even say "combo" instead of "combination", to be ultra-colloquial. :)