Colorless Vol. 4 (manga)

A cosmic disaster erased almost all colour from the world, and now the mutated descendants of humanity live in a moody, noir cityscape draped with shades of grey. There is great power hidden in what little colour remains, and a secretive cult known as the Order hopes to use that power to awaken their mysterious gods. Standing against them is Avidia, a gun-toting investigator and scientist who works outside the law.

If he's to prevail, he'll need more power - which means surviving an incredibly dangerous new experiment, even as the Order moves ahead with the consummation of all their schemes!