Screens mix colour by adding light — red and green light together make yellow. Paint does the opposite: each pigment absorbs certain wavelengths and reflects the rest, so mixing pigments removes more light than either alone. Naively averaging two hex codes (a common shortcut) actually models light-mixing, not paint — which is exactly why it turns blue and yellow into grey instead of green. This tool instead uses the RYB colour wheel — the model artists are actually taught — built from eight reference points (white, black, the three primaries, and their pairwise mixes) and blended between them, so mixing behaves the way a real palette does.
Mix Ultramarine Blue and Cadmium Yellow in equal parts here and you get a genuine green. Average their hex codes directly (#2A5F99 and #FFD400) and you'd get a muddy grey-blue instead — the naive-averaging failure mode this tool is built to avoid.
Every real pigment absorbs some light outside its "main" colour too — mixing pigments compounds those absorptions, which is why paint mixtures trend toward brown or grey the more colours you add, and why a "pure" digital blue looks more muted once it's modelled as a real pigment.
No — real pigments have their own reflectance curves (the basis of the more rigorous Kubelka–Munk theory used in industrial colour matching), and opacity, tinting strength and paint chemistry all affect the result. This is a reference approximation of subtractive mixing behaviour, not a colour-matching instrument.
A tint is a colour mixed with white (lighter, less saturated). A shade is a colour mixed with black (darker, often muddier since black pigment absorbs broadly). Both are separate from a tone, which mixes in grey.
Mixing colours from opposite sides of the colour wheel (like red and green) pulls the result toward brown or grey, since their absorptions overlap the least useful way — this is the same reason painters are taught to avoid complementary-colour mixes when they want a clean, vibrant result.