I think a 6" border of each color would overwhelm your prints. They are too cute to overpower. Possibly a first border of each color in 2" strips, then a 6" piano key border using the three colors and finish with a second border of each color in 2" strips. That way you would still have the width you needed but the borders wouldn't be as bold. BTW, I really love the fireman fabrics you've chosen and the border colors really work well with the firemen. Thanks for sharing.