Keep one colour, turn the rest black and white

Colour splash is not a separate effect here. It is the ordinary selection — a sampled colour with hue, chroma and lightness tolerances — with its weight inverted: pixels the selection holds keep their colour, and pixels it does not hold get the black-and-white treatment from the page next door. Because the weight is soft, a petal's pale edge is kept partly in colour rather than snapped to grey.

The coverage figure therefore reads both ways: the share of the frame kept in colour, and the share turned grey. A kept share that suddenly jumps means the sky or a wall has joined the selection; narrow chroma first.

1. Skin often sits close to reds and oranges. Turn on skin-like protection only if faces should stay in colour; otherwise narrow hue.
2. A highly compressed JPEG carries colour noise in grey areas, which a wide chroma tolerance keeps as speckles of colour.

How do I keep two colours instead of one?

Shift-click the second colour with the dropper. It joins the selection as an extra sample, shown as a chip under From, and both colours stay while the rest of the frame goes grey. Alt-click takes a colour back out.

The kept colour has a hard cut-out edge. Why?

Usually because edge softness is near zero. The splash is the selection's soft band turned inside out, so the feathered shoulder that keeps a recolour from looking cut out also lets the kept colour fade into the grey. Raise edge softness, or widen lightness so the colour's own shadow is kept with it.

Something else in the background is the same red and stays in colour too.

A colour selection cannot tell two red things apart. Use the Regions panel: draw a rectangle or paint “only here” over the object, and the colour is kept only inside it — everything outside goes black and white, including the other red thing.

• Make a photo black and whitePerceptual conversion, red to blue filter looks, a three-point curve and a true greyscale PNG.
• Replace a colourThe dropper armed on open, two swatches, one operation and nothing else to manage.
• Black and white channel filtersWhat a red, yellow, green or blue filter does to sky, skin and foliage, as weights.

Image data is never posted anywhere: decode, mask, remap and encode all happen inside this tab's memory, and that memory is released when you leave.