Simple and smart. From Terrie:
With two boys very close in age, we decided early on to "color code" a few items to simplify life and to be able to tell at a glance who the owner is. Son #1 gets blue, green or purple (cool colors), while son #2 gets red, orange, yellow (warm colors). (Son #2's middle name begins with an R, so we just think Red for him, and remember it.) This works for sippy cups, snack containers, toothbrushes, etc. — no more guessing who finished his snack and who still has some left! And allowing an assortment of colors for each rather than just one seems to ensure no one ends up totally dissatisfied.