I'm not sure but if you go looking in the controls API there is likely to be a SetColor() or something
and you will need to change the <OnMouseDown> to be inside each <label> At the moment it will run when you click the frame not each label individually. You will also need to give your function the name of which label you have clicked on so it knows which one to change, have a look at sending parameters to functions in lua for that
It's also a good plan to pre think things before you jump into coding. When I click this what exactly do I want to happen behind the scenes it saves a lot of time later
Most of what your looking for can be found in a quick google search
you'll learn a lot quicker and feel like you've accomplished more if ya go digging