Since Nokia3410, 3510i, 3520, 3530, 3560 and so on have the same size of screen, do you think if an application runs on one of models above can also be able to run on the others, provided the colors problem has been properly dealt with? I'm asking this question because I don't want to buy too many different models of phones to test my application.
You should really check what does term Nokia Platform mean.
I think there was a link about it right at the main page of Forum Nokia.
What do you mean saying "compatible".
Phones in same Serie have same screen size, and some other features. Check them. If your app fits series mandatory constraints, then it should be able to run ok on every serie phone unless there are some bugs in concrete phone. These bugs happen. Hence, so its desirable that final check was made on several phones of the serie (e.g., main target phones)