Most of the Bachelor/ettes have a purple, blue, green, and yellow flower event. To my knowledge only Dirk doesn't have any flower events but does have special requirements for marriage.
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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)
Dirk and Alisa don't have any flower events, but instead have specific requirements you must meet in order to marry them. Dirk, Alisa, the Oracle, and Mikhail do not have colored flowers; their flowers remain white like normal villagers, but they are indeed marriagable. Flower colors go like this: White-> Purple -> Blue -> Green -> Yellow -> Pink -> Red However there are only flower events for Purple -> Yellow, and you must view them all in order to marry that person