As I recall, the heart meter in the assets are a really general idea on how the person feels about you and isn't exact with the heart colors. (wow that sounds confusing x_x; ) To get to purple from black, you'll need to get him to 10k FP and since he likes any kind of fish, it'll take some time to get his heart up. Just take your time and talk with him and give him fish.
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~Harvest Moon Game List~
HMDSCute: Married to Kai Y2 HM - IoH: Married to Shea, Child, Pony(Y3) HM - SI: Married to Shea (again) (Y2) RF1 - Married to Mist (Completed game) (Y1) Currently Playing: ToTT: DS Farm: Stoof Name: Lily (Sum. Y2) (FC 0476-2982-0232) (PST)
Patience is required, just keep greeting Denny every day and giving small fish and you will reach Purple Heart.
Denny's favorite gift is Sashimi (800 points instead of 500 points for a small fish). Small fish are more readily available since Sashimi requires a cooking degree of 501 and a fishing degree high enough to catch medium fish. That also assumes that you have the recipe.
Greeting alone will give you 100 FP. Remember to greet Denny prior to giving him the fish. Giving a gift to a villager is also worth 100 points so greeting first will add an additional 200 points per day to your 500 point gift. Daily encounters are important.