What's a pixel?

Numbers.   A digital color image pixel is just a RGB data value (Red, Green, Blue). Each pixel's color sample has three numerical RGB components (Red, Green, Blue) to represent the color. These three RGB components are three 8-bit numbers for each pixel. Three 8-bit bytes (one byte for each of RGB) is called 24 bit color. Each 8 bit RGB component can have 256 possible values, ranging from 0 to 255. For example, three values like (250, 165, 0), meaning (Red=250, Green=165, Blue=0) to denote one Orange pixel. Photo editor programs have an EyeDropper tool to show the 3 RGB color components for any image pixel.

In the base 2 binary system, an 8 bit byte can contain one of 256 numeric values ranging from 0 to 255, because 2 to the 8th power is 256, as seen in the sequence 2,4,8,16,32,64,128,256. The 8th of these is 256. This is the same concept in base 10, that 3 decimal digits can store one of 1000 values, 0 to 999.   10 to the 3rd power is 1000, same idea as 2 to the 8th power is 256.

Yeah, right, but the only point here is that 255 is the maximum possible number that can be stored in an 8 bit byte. Larger numbers require multiple bytes, for example two bytes (16 bits) can hold up to 256x256 = 65536 unique values. 24 bit RGB color images use 3 bytes, and can have 256 shades of red, and 256 shades of green, and 256 shades of blue. This is 256x256x256 = 16.7 million possible combinations or colors for 24 bit RGB color images. The pixel's RGB data value shows "how much" Red, and Green, and Blue, and the three colors and intensity levels will be combined at that image pixel, at that pixel location.

The composite of the three RGB values creates the final color for that one pixel area. In the RGB system, we know Red and Green make Yellow. So, (255, 255, 0) means Red and Green, each fully saturated (255 is as bright as 8 bits can be), with no Blue (zero), with the resulting color being Yellow.

Black is a RGB value of (0, 0, 0) and White is (255, 255, 255). Gray is interesting too, because it has the property of having equal RGB values. So (220, 220, 220) is a light gray (near white), and (40,40,40) is a dark gray (near black). Gray has no unbalanced color cast.

Since gray has equal values in RGB, Black & White grayscale images only use one byte of 8 bit data per pixel instead of three. The byte still holds values 0 to 255, to represent 256 shades of gray.

Line art pixels are represented by only one binary bit with values 0 or 1, used to denote Black or White (2 colors, no gray). Line art data is stored packed 8 bits into one 8-bit byte.