When my daughters were little, they wore a lot of the same pieces of clothing and were pretty close in size. I would find an outfit that was so cute, but did not want to purchase two in the exact same color for sisters. To keep their clothing straight, a lot of the time I would purchase pink for my youngest daughter and light blue for my oldest daughter. The GAP made this easy because most outfits would come in both color combinations. This really helped in sorting laundry. At one time, I even monogrammed their little white Nordstrom undergarments in an effort to keep everything straight.
This effort has of color coding my kids has continued with my son who is coded "green". This really comes in handy for water bottles, etc. We still have items monogrammed (sports bags from LL Bean are navy with each child's monogram).
I love these water bottles because you can put them in the freezer and the outside fluid freezes and keeps the water bottle cold.