IBM PCjr Manual page 82

Function Mode
The border around the Function (Fn) key is green.
When you press the Fn key, the keyboard waits for you
to press one of the keys with a green stripe underneath
it. The number of the function each key performs is
shown in the green stripe underneath the key. The
program you are using determines what that key does.
To use the Function mode, first press and release the
Fn key. Then press the green striped key you desire.
After you press the green striped key, the Function
mode is canceled.
If you have more than one function to perform, you
may press and hold the Fn key while pressing the other
green striped keys.
3-22 Keyboard