ON harvest moon Ds why does Nami's heart change from blue to purpel ?

If it went from blue back to purple then that means you went down a level of affection. (the order is black,purple,blue,green,yellow,orange, red) If you are trying to marry Nami then that's bad if your marrying someone else then it doesn't really matter. You could have lowered her affection in a number of ways littering, giving her a bad gift, or maybe you said something wrong during a heart event. If you want to raise her heart again make sure to talk to her everyday and find gifts she likes then giver her those.