Underwater, Red Is the First Color to Leave
Each metre of water takes a slice of the spectrum, and the sea turns blue not by adding anything but by subtracting.
Seawater is almost clear in a glass and distinctly blue in a pool, because water absorbs light slowly and unevenly. Red goes first, mostly gone by five metres; orange and yellow follow within fifteen; green holds out longer, and blue, the last survivor, is what reaches a reef at thirty metres and what comes back up to the surface. A diver's cut looks green down there and a red fish looks black, until a torch puts the missing colors back.
This is why underwater photographs come in two kinds. The ones taken with available light are a study in blue, everything beyond arm's reach fading into the same soft dusk, the surface overhead a bright, rumpled ceiling. The ones lit by a strobe close to the lens are suddenly loud, the coral orange and the sponge crimson, colors that no one has seen in place since the reef grew. Both are true; they differ only in how far the light had to travel through water before and after it met the subject.
For a screen the available-light kind is the easier company. Its range runs from navy to pale cyan with almost nothing warm to argue with white text or an orange folder, and the surface light arriving from above gives the image a natural top edge where the menu bar can sit. The eye settles into it the way it settles into deep shade. Whatever you place over it will be, by a wide margin, the reddest thing in the picture.
