You ask- Personally I don't like the orange in this quilt- I think it really takes away from the other two colors. But I don't care too much for adding pop colors either. I you must add another color make it a softer color perhaps a light orchid to go with the soft mint and gray. Orchid and mint go well together maybe light mauve I personally like the grey and green, perhaps you could add another color for the binding. I think I would ask daughter and see what she thinks. I love mint green for babies.
Our quilting group did our annual challenge quilt and it was to be black and white with a pop color. I had more trouble doing this quilt, because to me a black and white quilt is just that, black and white with no other color. I finally added burnt orange and gave it to my son for Christmas - big U-TX. fan and this worked out o.k.