Duotone

A printing trick before it was a filter: run a grayscale photo through two inks instead of one and you get more tonal depth plus a color cast. Two inks does not mean two colors on the page. Where the inks overprint at different densities you get every mixture between them, so the picture keeps its continuous tone. Digitally you strip the color to pure luminance, then map that ramp between two endpoints, the dark one landing in the shadows and the light one in the highlights, with smooth interpolation everywhere in between. Photoshop has a real Duotone mode for the print workflow, most people simulate it with a Gradient Map, SVG does it properly with feComponentTransfer, and CSS fakes it with a stacked grayscale plus multiply plus screen sandwich.

It earns its keep when a pile of mismatched stock or user photos has to look like one brand. Duotone flattens the differences between them, drops the detail that competes with overlaid text, and locks everything to your palette. Spotify built an entire identity on it, and conference sites and gig posters have leaned on it for decades.

Gotcha: pick two colors with real distance in brightness, not just in hue. Two equally bright colors collapse the image into mud. And any information the original carried through hue alone, a red jacket against green grass, vanishes completely once you flatten to luminance, so always check faces and product shots before shipping the treatment. Last one: do not confuse this with posterization. Duotone constrains the palette while the tonal ramp stays smooth, posterization chops the ramp into a handful of flat steps.