01 · Two colours, thirty counts apart, twice
What the number under the word tolerance is measuring, and why the same setting behaves as two different settings inside one photograph when that measurement is taken in RGB.
Read it if a selection you were happy with in the highlights fell apart in the shadows.
02 · Chroma near zero, and the hue angle that is noise
The chroma floor stated as a number, and what each of the three neutrals will accept: a white shirt takes a colour, a mid grey takes one convincingly, a black jacket takes almost nothing.
Read it before promising somebody a yellow version of a dark garment.
03 · Chunk by chunk, and the one that changes
The one family of files this site recolours without reading a pixel: at most 768 bytes of table rewritten, one checksum recomputed, and a saved file the same size as the one that arrived.
Read it if what you are recolouring is a logo, a chart or a screenshot rather than a photograph.
04 · Asked, got, ΔE_OK, four times
Kept lightness, the chroma scale and the sRGB gamut: the three reasons the delivered colour differs from the typed one, shown on four receipts the engine computed.
Read it before telling a client their brand colour is on the photo.
05 · Coverage before and after a rectangle
Why no tolerance separates a blue jacket from a blue sky, and how a region, a shape or the person finder fences the selection instead.
Read it when the background keeps joining in.
06 · One ramp, three modes
The three remap modes on one painted ramp, with the chroma spread and lightness range each leaves behind.
Read it if a recolour came out looking like paint.
07 · Four subjects, six presets
Red, orange, yellow, green and blue filters as the channel weights they are, with the grey each gives sky, skin, foliage and a red flower.
Read it before converting a landscape or a portrait.
08 · The colour rules, with their sources
Identity photographs must be unaltered, and exam forms state colour rules for photos and signatures; what a recolour tool must refuse to promise.
Read it before touching a passport, visa or exam photo.
Every argument above is checkable in about thirty seconds on the instrument itself, and the coverage figure under the tolerances is the thing that tells you whether a page was right.