Research has shown babies brains are more stimulated by bright primary colors than pastels, so I would expect that would also be the case that it would be more stimulating than for greys. Plus what if the little girl grows tired of pink, and they then paint the room a different color.
You asked the mother, but as the creator I think you would also have some say in your creation. And if you ask me the mother's reason is rather weak in making a blanket for one child to fit in with a visiting child. The baby will be using the blanket all the time and the baby won't care what coordinates with the wall paint.
I would vote for making something the baby would like long term.