Android uses standard RGB (red, green and blue) color model. Each primary color value is usually represented by hexadecimal number. At the beginning of such a color definition you have to put a pound character (#).
The simplest is just #RGB format, where #000 is black and #FFF is white. But in this format we have only 16 values per color so it gives 4096 combinations. That’s why #RRGGBB format is mainly used. In this format we have 256 values per primary color, so 16 777 216 colors in total. Black is #000000 and white is #FFFFFF.