Regardless of colour I don't actually like those PJs anyway.
I would write a very nice thank you note
"dear Auntie xxx, thank you so much for the pink giraffe pyjamas which you sent me for my 1st Birthday. It was very thoughtful of you.
Lots of love
Little Santa"
And then I would quietly exchange them or re-gift them.
I exchanged lots of clothes given to me when my two were small either because I didn't like them or because we had too much of that item or size. I always thanked the giver nicely and I never mentioned it.
Just wait next year she'll buy him the pink car garage from ELC.
The year after that he'll be old enough to express his own opinion on her gift choices.