
Sonia,
I'm sure you'll get different responses to this, but yes in my (second hand) experience there are differences in temperament between the different Dane colours. (A friend of mine breeds blues, blacks and harlequins.) Harlequins are usually found to be a bit 'dafter' than the other colours, and from my own experience this follows through in the equivalent merle colouring in Collies.
However, it stands true that in all colours there will be 'lines' or families that are not known for their good temperaments! Don't know which of the 5 colours (or 7 if you include the harlequin mis-marks) you're looking for - but if you want any advice on lines that are known for good temperament, drop me a line and I will check it out with my friend!! Let me know if you just want a pet, or if you're looking to show/breed.
As far as what else you need to ask breeders goes - there's loads!! Obviously, as with all giant breeds, the Danes are often not particularly long-lived but you can increase your chances by checking out your breeders with regard to health and longevity. If a breeder tells you there are no health problems within the breed, they are lying and I would suggest you look elsewhere. Best thing to do is canvas several breeders before going to look at puppies. Major health problems include cardiomyopathy (heart) and 'Wobblers' syndrome which usually results in dog having to be put to sleep early.
As I say, if you'd like some advice and suggestions of reputable, health-conscious breeders, drop me a line with a normal email address and I'll put you in touch with my friend.