Do the different coloured tiles give different items...?

are you asking if there's a correlation between the color of the tile and what you get? Not that I can tell. It appears to be a fairly good random distribution.

If other colored tiles give you a different item for that particular day (that is to say does it matter at all which one you pick), there's no way anyone would know.

I always pick the purple one and I get each time another item.

Yeah, like maybe only certain pets can be found under certain tiles, just curious. I did assume it's all random though.

Haha.. I normally pick the purple tile and I seem to get a crazy amount of Festers and Darkonites.. maybe its just coincidence then. :)