颜色的种类

https://www.wisegeek.com/how-many-colors-can-humans-see.htm

There are an estimated 10 million color hues that humans can see. The human eye has three cones, each of which is able to view either red, green or blue. All of the other colors that humans can see are referred to as secondary colors and are created by combinations of these primary colors. It generally is believed that the human eye cannot see red-green or yellow-blue, referred to as the forbidden colors, because each color in the pair takes different actions in the brain that cancel out each other.

More about color vision:

The most fatiguing color to the eye is thought to be bright pure yellow, because the color reflects the most light and thus causes the most stimulation of the eye.
Chickens that are exposed to red light are less aggressive, studies show, although it is not known why this happens.
In 2010, researchers found the first confirmed tetrachromat — a person who has four cones in the eye instead of three, and who might be able to view hundreds of millions of colors. 正常人类有三种视锥细胞，是三色视者；大多数的色盲患者和其他哺乳动物只有两种视锥细胞，为双色视者；某些特殊的人群则有四种视锥细胞，被称为四色视者。

关于CIE色度图

The CIE 1931 RGB color matching functions

CIE chromaticity diagram

上面边界的曲线是和单色光一一对应的，而下面边界的这条直线却没有单色光对应，而是两个光谱色的混合色轨迹。因为它是红色和蓝色的混合，所以主要是紫色的。这意味着，没有单色光形成的紫色。所有带紫的颜色，什么紫红色，品红色，都是所谓的 谱外色。

可见光光谱波长从长到短是红、黄、绿、蓝、紫。而品红色介于红色和蓝色之间，是由等量的红光与蓝光混合而得。就是说，在光谱中，品红色并不是单一波长的光。除非把两种波长的光混合在一起，而且两种光的强度还要合适，否则在连续光谱中不可能存在品红色光。

Non-spectral colors

Among some of the colors that are not spectral colors are:

Grayscale (achromatic) colors, such as white, gray, and black.
Any color obtained by mixing a gray-scale color and another color (either spectral or not), such as pink (a mixture of a reddish color and white), sky blue (a mixture of blue and white) or brown (a mixture of orange and black or gray).
Violet-red colors, which in color theory include line of purples colors (such as, approximately, magenta and rose), and other variations of purple and red.
Impossible colors, which cannot be seen under normal viewing of light, such as over-saturated colors or colors that are seemingly brighter than white.
Metallic colors which reflect light by effect.