I don't think this mysterious creative decision for Link's hair color in A Link to the Past has ever been officially disclosed by Nintendo. Why is his hair pink? It never appeared that way in any of the promotional artwork.
All I've heard is a bunch of speculation, such as it somehow being that way palette-wise to accomodate for the bunny form in Dark World, or due to some kind of technical limitation with sprite colors (both which I'm pretty sure have been debunked by rom hack edits by now).