Purple versus violet

The color terms purple and violet cause confusion for many people. This is because the two color categories overlap to a considerable extent, and different cultures use the two terms in different ways. Many people think of the terms purple and violet as having no difference at all.

In many contexts, such as art, the terms are fairly straight-forward, however. Purple is a color intermediate between red and blue which veers more towards the red part of the spectrum. Violet, on the other hand (and, as the color of the flower suggests), veers more towards blue. Purple also tends to be a richer, more saturated color. In RGB terms, purple actually tends to contain fractionally more red, but also considerably more blue. In CMYK terms, purple contains more black but less cyan and magenta.

The two terms are also different in a psychophysical context. On a chromaticity diagram, purples lie along a line connecting the extreme colors red and violet, and the color is thus located between the two in hue. It lies closer to violet than do cerise, crimson, madder, magenta and other pinkish-reds. Violet, by way of contrast, lies closer to blue than purple, but not as close as indigo.

In a color circle, violet's wavelength (around 440nm) is seen within the visible spectrum, at the extreme blue end. Purple does not lie within the spectrum as such (although, obviously, it is visible), but is rather the admixture of the colors at the two ends of the spectrum (red and violet). As such, it lies in the color wheel's "gap" - at extraspectral region representing hues that in themself do not have a unitary wavelength specification.

One interesting psychophysical feature of the two colors which can be used to separate them is their appearance with increase of light intensity. Violet, as light intensity increases, appears to take on a far more bluey hue as a result of what is known as the Bezold-Brücke shift. The same increase in blueness is not noted in purples.