When a Function key has two jobs to do, one is the icon printed the same colour as is the Fn key itself. An example could be Function 4 for wi-fi has the wireless icon in blue. If blue is the colour on the Fn key, you need to press both keys together.
Whatever the other function on, say F4 again, is Mute sound and is in black, just pressing the key without the Fn It almost doubles the number of keys. F5 is usually reserved because it refreshes the page on an Internet site.