The purpose of the color model (also known as the color space live color system) is to describe colors with acceptable resolution under certain standards. Generally, the definition of color space is hardware-oriented or application-oriented. Common ones are RGB, CMY, CMYK, HSI, CIE, YIQ, YUV, etc.

1.RGB (red, green, blue red, green and blue) color model
In the RGB model, each color appears in the primary color spectrum components of red, green and blue, and all colors are composed of red, green and blue. Based on Cartesian coordinate system.
Assume an RGB image, each channel is 8 bits, under this condition, each pixel has 24 bit depth.Full color imageUsed to represent a 24-bit RGB color image. In a 24-bit image, the total number of colors that can be displayed is (28)3=16777216 $(2^8{)}^3=16777216$.
is mainly divided into the following types:
（1）RGB555
RGB555 is a 16-bit RGB format, each component is represented by 5 bits, and the remaining one is not used.
high byte -> low byte
XRRRRRGGGGGBBBBB
（2）RGB565
RGB565 is also a 16-bit RGB format, but R occupies 5 bits, G occupies 6 bits, and B occupies 5 bits.
（3）RGB24
RGB24 is a 24-bit RGB format, each component occupies 8 bits, and the value range is 0-255.
（4）RGB32
RGB24 is a 32-bit RGB format, each component occupies 8 bits, and the remaining 8 bits are used as Alpha channels or not.
pay attention:*Stable RGB color set, Or calledSystem-wide stable color collectionIt is a subset of the RGB color space. Called in the internetStable WEB color or stable browser color*
The stable RGB color set also consists of three RGB channels, but each channel can only take a few fixed values. Such as 1,51,102,153,204 or 255.

2. CMY(cyan,mayenta,yellow, crimson, yellow) and CMYK(CMY and black, add black channel)
Cyan, deep red and yellow are the secondary colors of light and are the colors of paint. More data input is used for printers and copiers.
The primary pigment colors of the same specific gravity, cyan, deep red, and yellow. Black can be generated, but the black is not pure. In order to generate a pure black, K (black) is added and the CMYK model is proposed.
3. HSI (hue, saturation, brightness)
The mutual conversion between the RGB model and the CMY model is relatively simple, but it does not fit well with the colors explained by humans.
Hue: [1] Hue is a color attribute that describes a solid color
Saturation: [1] Saturation is a measure of the degree to which a pure color is diluted by white light
Brightness: [1] Brightness is a subjective descriptor, which is actually not measurable
4.CIE()
Introduction to CIE Color System According to the CIE color system adopted in GB/T 7707-1987 "Gravure Decoration Prints", the following introduction is given.
The CIE color system is based on the three primary colors of red (R), green (G), and blue (B), which are used to determine the color mixing ratio; usually the color stimulus is determined by the additive color mixing method The principle to achieve.
5.YIO()
YIQ, is NTSC (National Television Standards Committee) television system standard. Y is the luminance signal (Luminance) that provides black and white TV and color TV, that is, brightness, I stands for In-phase, the color ranges from orange to cyan, and Q stands for Quadrature-phase, and the color ranges from purple to yellow-green.
6.YUV()
YUV (also known as YCrCb) is a color coding method adopted by the European television system. In modern color TV systems, three-tube color cameras or color CCD cameras are usually used to capture images, and then the color image signals obtained are separated, amplified and corrected to obtain RGB, and then the brightness signal Y and two are obtained through a matrix conversion circuit. There are three color difference signals RY (i.e. U) and BY (i.e. V). Finally, the transmitting end encodes a total of three signals of brightness and two color differences, and transmits them on the same channel. This color representation method is the so-called YUV color space representation. The importance of using the YUV color space is that its luminance signal Y and chrominance signals U and V are separated. If there is only Y signal component and no U, V signal components, then the image represented in this way is a black and white grayscale image. Color TV adopts YUV space to solve the compatibility problem between color TV and black and white TV with brightness signal Y, so that black and white TV can also receive color TV signals.
7. Conversion

RGB~CMY: (Assuming that the color values ​​are all normalized to the range [0,1], generally normalized for convenience)

RGB~HSI

HSI~RGB

RGB-CIEXYZ

RGB-YIO

RGB-YUV

References: [1] Digital Image Processing Gonzalez 428~429;