We generally use either six character Hex code to get the color or RGB values. Although these are the appropriate way to show the color. One other way to have colors is three character hexadecimal codes. Coming to three characters we limit only 4096 colors. Based on our common perception, these 4096 colors are sufficient to do any color based operation. These codes are called Shorthand notation of hexadecimal codes.
Remember using three character code in bgcolor="#ABC" might not give the correct appearanace as it makes that back to six character code which is #ABC000 and which is basically #AC0 when converting to shorthand notation. Therefore, use CSS property style='{background-color:#ABC}'.