ShotMarker magnetic sensor brackets mount in the corners of your target frame to hold a sensor in the correct orientation.
A set of four includes red, orange, green, and blue, and a set of #8 x 1" wood screws, which will allow you to set up an additional target frame.
Functionally, all colors are exactly the same. The white bracket is a handy spare since it can replace any color with the benefit of being less confusing than the wrong color.