his hair turns white because he is blessed, other sirens are more colorful and can be naturally pink, blue, bright green, similar to normal birds but particularly a Kingfisher. Their hairs and wings can be multicolored. Only blessed sirens have pure white feathers as blessed sirens are actually sirens who died but came back to life (thus blessed by life). Sirens that die will lose their color in their hair and feathers, turning pitch black
All sirens have a different hair color and eye color when in their human form as a survival mechanic from prey but for Irwin because he is blessed, he has a harder time hiding his natural eye color.