As a child, Arthur only saw pink three times: on a rose, on a little girl, and on his sheets once when they were washed with his father's cape.
After that, he decided he didn't like it. He associated it with all things girly and frilly.
Guinevere's dress didn't change his mind: it was girly.
But he didn't mind. That was part of what made it so perfect.
And five years later, when Guinevere handed him a pink bundle in a pinker blanket… he decided it may be his favorite color.
Not that he would say that aloud or anything.