Why are the submarines painted black?

Modern submarines are painted black as it's the easiest color to maintain, provide camouflage from aerial observation while at periscope depth (e.g., no reflecting surfaces), and still give enough profile color while surfaced in daylight to provide visibility to other ships in navigation lanes.

However, under certain wartime conditions, submarines are repainted and camouflaged like other naval vessels with broken paint camouflage schemes to make them nearly impossible to see near the surface or when surfaced.