Answer:
If you keep giving the person gifts, it will definitley change colour. It kinda depends how long it will take, for example, if you were to give Ann Truffle Rice, her LP (love points) would go up 800. If you were to give her something she absolutley HATES, then her LP would go down a whopping 1000LP. So be careful! type in this link if you need any more help.
http://www.fogu.com/hm4/