Colour difference calculator
Perceptual difference ΔE 4.7 — noticeable side by side
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Measures the perceptual distance between two colours as ΔE in CIE Lab; the space designed so that equal numeric distances look like equal differences.
How to use the colour difference calculator
RGB distance is the obvious approach and it is wrong: it treats a change in blue as being as visible as the same change in green, and human vision is far more sensitive to green. Lab was built to fix exactly that. The thresholds worth remembering are that ΔE under 1 is not perceptible to the human eye, 1 to 2 is perceptible only on close inspection, 2 to 10 is perceptible at a glance, and above 50 the colours are more different than they are alike. This uses the 1976 formula, which is simple and sufficient for the verdict; CIEDE2000 is more accurate for saturated blues if the number itself matters.
Questions
The threshold of perceptibility. Under 1 the human eye cannot tell the two apart.
Because RGB distance does not match perception. It treats a blue change as equal to a green change, and your eye does not.
CIE76, the straight-line distance in Lab. Simple and adequate for a verdict; CIEDE2000 is more accurate in saturated blues.
Commonly under 2 for a brand colour match, which is around the limit of what a client notices side by side.
Hex for anything going into a design tool, HSL when you want to adjust lightness by hand, and OKLCH when you need two colours to look equally light; it is the only one of the four where that number means what you expect.