The color of the birthday set in the account profile cannot be changed. If you wish to change the color to your liking, please follow these steps:
A) Hide your birthday from the calendar view from profile settings
- Tap the image icon at the top left corner
- Tap your account at the top of the screen
- Tap "Edit Profile" below your profile icon
- Set "Birthday Display" to [Hide]
B) Create a yearly repeating event on your birthday with the label color you wish to use.