Colour Splash
One colour stays, everything else turns to greyscale. It is the red-dress-in-a-black-and-white-street effect, and it works by comparing each pixel's hue against a target: inside the tolerance the colour is kept, outside it the pixel is desaturated, and the feather zone between the two prevents a hard cutoff that would look like a bad selection.
Drop images here
PNG, JPG, WebP — multiple files allowed
Why the saturation floor exists
Hue is meaningless for a grey pixel. A neutral wall has a numerical hue — whatever tiny imbalance happens to exist between its channels — and if that number lands near your target, whole areas of supposedly grey image will be "kept". You see this as random patches of faint colour surviving across the background.
The saturation floor solves it: any pixel less saturated than the threshold is treated as grey regardless of its hue. Fifteen percent is a good default. Raise it if you see stray colour in areas that should be neutral; lower it if pale but genuinely coloured regions — a pastel shirt, distant sky — are being wrongly desaturated.
Tolerance, feather and the red problem
Tolerance is measured in degrees around the colour wheel, in both directions, so 30 accepts a 60° band. Under about 15 the selection becomes brittle and misses shaded parts of the same object, because shadow shifts hue as well as brightness. Over about 60 you start catching neighbouring colours — with red as the target that means oranges and skin tones, which is the single most common way this effect goes wrong. If you are isolating red on a photo with people in it, keep the tolerance low and expect to accept some loss.
Feather extends past the tolerance with a linear ramp from kept to grey, so an object's edge fades out rather than stopping dead. Fifteen degrees is usually invisible in the right way. Set it to zero for a hard, poster-like selection.
The boost multiplies the saturation of whatever survives. A little — 20 to 50% — makes the kept colour hold its own against the grey surroundings, which is genuinely necessary since the eye reads an isolated colour as weaker than it is. Past 100% it goes garish quickly.
FAQ
How do I get the exact target colour?
Use Color picker on the image first — click the pixel you care about and it gives you the hex, which you paste in here. Picking from a shaded part of the object usually generalises better than picking from a highlight.
Parts of my object went grey.
Its hue varies more than the tolerance allows — usually because of shadow or a specular highlight. Raise the tolerance and the feather, and lower the saturation floor. There is a limit: this selects by colour, not by object, so a red thing lit by blue light is not entirely recoverable this way.
What does invert do?
Exactly the opposite: the matched colour is desaturated and everything else keeps its colour. Useful for removing a distracting colour — a bright exit sign, a stray logo — without touching the rest of the frame.
Is this the same as Change color?
No. Change color replaces one colour with another. This one keeps a colour and drains everything else, which is a selection problem rather than a substitution.