Academic painting in 19th-century France favoured smooth, carefully blended colour, mixed on the palette until it matched the exact hue an artist wanted before it ever touched the canvas. The painters who came to be called Impressionists — Monet, Renoir, Pissarro among them — largely abandoned that approach, applying separate strokes or dabs of relatively pure, unmixed colour directly next to each other and relying on something outside the paint entirely to finish the job: the viewer's own eye.
Optical mixing: blending that happens in the viewer, not the paint
When small patches of two different colours sit close enough together, and are viewed from a normal viewing distance, the eye can't fully resolve them as separate patches — instead it perceptually blends them, producing a sensation of colour that neither pure pigment alone actually contains on the canvas. A dab of pure yellow next to a dab of pure blue, seen from across a room, can read to the eye as a vibrant green — a green that's often more luminous and lively than the same green would look if it had been physically mixed on the palette and applied as one flat, uniform patch. Impressionist painters exploited this deliberately, particularly to capture the shifting, luminous quality of outdoor light, painting shadows in unexpected colours like violet or blue rather than the flat grey or black academic convention favoured, because they'd observed that shadows in natural daylight really do carry colour, not just an absence of light.
A technique later painters pushed even further
A younger generation of painters, most notably Georges Seurat, took the same underlying idea and made it far more systematic. Pointillism (also called divisionism) applied the principle of optical mixing with rigorous, near-scientific discipline, building entire canvases out of thousands of tiny, individual dots of pure colour, carefully calculated to blend in the viewer's eye into the intended overall hue and tone when seen from the correct distance. Where Impressionist brushwork was relatively loose and improvisational, Seurat's method treated optical mixing almost as an engineering problem, drawing directly on contemporary colour theory and research into how the human eye actually perceives colour.
What we're still unsure about
The basic physiological mechanism behind optical mixing — the eye's limited ability to resolve very small, closely spaced areas of colour at a distance — is well understood and grounded in visual science, not a live dispute. What art historians still actively debate is how consciously and precisely 19th-century painters were applying formal colour theory versus working from accumulated visual instinct and trial-and-error observation; Seurat clearly engaged with contemporary colour science quite directly, but how deliberately earlier Impressionists like Monet were reasoning about optical mixing, as opposed to simply painting what looked right to them, remains a genuinely contested question without a clean, documented answer.
This sits inside Impressionism & Colour Relationships, one of seven topics in Painting, one of five domains in Art, one of seventeen subjects the app can quiz you on.