Every child learns red + blue = purple, and every painter learns the hard way that it usually makes grey-brown sludge. The theory is fine; the pigments are the problem. Getting clean purple is entirely about choosing the right red and the right blue.
Why your purple turns grey
Most reds lean orange (cadmium-type fire-engine reds) and many blues lean green (cyan-types). Orange and green are both purple's enemies: they sit across the wheel from it, so any orange or green bias in your starting paints mutes the mix. Fire-engine red + green-leaning blue contains all three primaries at once — and three primaries make brown, as covered in what colors make brown paint.
The pairs that actually work
Magenta + blue is the clean route. A pink-leaning red — magenta, fuchsia, rose — carries no orange, so it mixes vivid purples with almost any blue. This is the closest thing to a cheat code.
Crimson + ultramarine-type blue is the classic painter's pair: a cool, blue-leaning red with a warm, red-leaning blue. It gives rich royal purples and plums rather than neon violet.
Avoid warm scarlet reds and green-leaning blues for purple work — they can only make muted mauve at best (which, to be fair, is sometimes exactly what a moody cloak needs).
Steering the result
- Toward violet/blue-purple: more blue, or start from ultramarine.
- Toward plum/red-purple: more of the cool red.
- Lavender/lilac: add white — purple takes white gracefully, unlike brown.
- Deep royal shadow: add a touch more blue rather than black; black turns purple to charcoal fast.
- Muted, greyed purple on purpose: a whisper of yellow — its complement — knocks the saturation down evenly.
Check the mix before you commit
Whether your particular red and blue will sing or sulk depends on their actual measured colors, not their names. Pick both bottles in the paint mixing calculator and see the result before wasting paint. And since every major brand sells purples ready-made, it is worth checking whether the shade you are chasing already exists — every purple across twelve brands is swatched in the charts, starting from the Vallejo color chart and the Citadel paint chart.