We want study if 2 proteins A and B are co-located, for that we use 2 FTP(Fluorescent tag proteins) for each protein?and after the expirement these 2 FTP are co-located. Does that mean necessarily protein A and B are co-located?
"Are protein A and B co-located?" The answer is that it depends. Since your test question included the word "necessarily", this means that the answer to the test question is no. All you have to do is think of at least one plausible counter-example.
Here's a couple:
The flourescently tagged constructs were screwed up somehow, so A, B, and each of their respective tags were actually expressed as 4 separate proteins.
(As per BPinto's comment) the tagged constructs may co-localize even if A and B would not normally do so. This can happen if, for example, the tags bind to each other, or if the tags somehow cause both constructs to get transported to a particular organelle.