I live along the NJ coast line and purchased 3 burgundy mandevilla plants. They are not the climbing type and they are not mounding type. They seem to just stand upright about 18″-20.” Planted them after Memorial Day and they were doing fine. They receive full sun for about 4 hours a day. However, in the past week the new blooming flowers are blossoming white. Not a nice white. A sickly white. The buds look burgundy before they bloom. There are no yellow spots and the leaves look dark and green. I use a watering can to water them once a day and have not yet fertilized.
I'm sorry your Mandevilla isn't doing well! There are a number of issues that this could be, but this article may help you: it describes some common diseases that Mandevilla get, and how to treat them (if they can be treated):