In the marine world, when your back is dark and your belly is light, you are using a camouflage technique known as countershading.

An animal swimming above you will not see your dark back as it will blend with the sea floor, or the unlit marine depths. While an animal swimming beneath you will confuse your white belly for sunlight above.

Countershading is a technique also employed by some sharks, fishes and seabirds such as penguins and puffins, and enables you to avoid detection. Either to avoid being someone’s meal, or it can improve your chances of snagging your next meal! Eat or be eaten!