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.
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.