Images are created by defining a grid of dots, called pixels. Each pixel has three numbers that define the color (red, green, and blue), and the grid is created as a list (rows) of lists (columns).
I've heard only three colors (RGB) are needed because these colors are enough to recreate what the human eye perceives. I think it’s interesting how such a wide range of colors and detailed images can be created just by changing the intensity of three simple values in each pixel.