Keep one colour and turn the rest of the photo black and white. Pick the colour from a list found in your picture, set how wide a match counts, and soften the edge.
Compares every pixel with the colour you chose and measures how far apart they are in red, green and blue. Pixels within the tolerance keep their colour; everything else is converted to grey using the same brightness weights as a black and white photo, so the picture keeps its tonal balance. With Soft edge above zero the boundary is a gradual fade rather than a hard cut, which hides the jagged outline that a simple keep-or-drop rule produces on JPEG compression artefacts.
A 4000 × 3000 street photo with a red London bus in a grey, rainy scene. The tool suggests the bus red as the first chip; at tolerance 60 the bus body stays red but the darker wheel arches go grey, so tolerance 95 brings them back while the brake lights of the car behind, a more orange red, are still excluded. Soft edge 20 removes the halo around the windows. The result, saved as JPEG at quality 88, is about 1.6 MB, a little smaller than the 2.1 MB original because grey areas compress better.
Colour splash draws the eye to one subject: a bouquet, a door, a jersey, a single balloon. It works best when the chosen colour appears nowhere else in the frame. It struggles with skin tones and with colours that also occur in the background, such as green on grass, because everything within tolerance stays coloured. If you want the whole picture in black and white use Grayscale Image; if you only want colours toned down rather than removed, Change Saturation is gentler, and Adjust Hue can shift the kept colour afterwards.
Keep this colour is the classic effect; Grey out this colour inverts it. The Colours in this photo chips are computed from a small copy of your image and skip near-greys. Tolerance is the maximum RGB distance, 0 to 255 scale; 40 to 80 covers one object with its shading, 150 covers a whole colour family. Soft edge is the width of the fade band in the same units. Output PNG keeps transparency if the source had it.
How do I choose the exact colour from my photo? When the picture loads the tool lists the eight most common colours it found, skipping greys. Tap one of those chips and it becomes the target; you can then fine-tune it with the colour picker or widen the tolerance.
Why is part of my object grey? Shaded or highlighted parts of an object are a different shade of the same colour, and the tool measures distance in RGB. Raise Tolerance until the whole object is included, then raise Soft edge a little to blend the boundary.
Can I do the opposite and remove one colour? Yes. Switch to Grey out this colour and only pixels close to the chosen colour lose their saturation, which is handy for muting a distracting red sign in a landscape.
Does anything get uploaded? No. Colour matching and the grey conversion both run in your browser. The photo never leaves your device.