What is the main color pattern called when a penguin has a dark back and white belly?

The dark-back, white-belly pattern of penguins is called countershading.

Countershading is a form of camouflage in which an animal is darker on the upper surface and lighter underneath. Viewed from above, a penguin’s dark back can blend with deeper water; viewed from below, its pale belly can blend with the brighter surface or sky.

This pattern is common among aquatic animals, including many fish, sharks, seals, and whales. In penguins it helps reduce contrast while they swim, although it does not make them invisible.

Countershading is different from the bold chest bands found on some banded penguins. Those bands are markings within the overall plumage pattern, while countershading describes the broad light-versus-dark arrangement across the body.