From MSDN:

A Color object stores a 32-bit value that represents a color. The color value contains four, 8-bit components: alpha, red, green, and blue. The first 8 bits (the most significant) contain the alpha component, the next 8 bits contain the red component, the next 8 bits contain the green component, and the next 8 bits (the least significant) contain the blue component. The 32-bit value is stored in a variable of type ARGB.