Newbie question: Button help

I am new to actionscript 3. I have created buttons with up/over/down states. The button changes colour during the over/down frames.

How do I keep the over or down state active until the user clicks another button?

Right now when the user puts the mouse over the buttons, they change colour, but it goes back to the original colour once they click off. I would like it to remain on the changed colour so that the user remembers which button s/he’s pressed until the another one is clicked.
I came up with the following code in actionscript 3 and got it to partially work:

ActionScript Code:
[LEFT]btn1.[COLOR=#000000]addEventListener[/COLOR][COLOR=#000000]([/COLOR]MouseEvent.[COLOR=#000000]MOUSE_UP[/COLOR], startTransparent[COLOR=#000000])[/COLOR]

[COLOR=#993300]function[/COLOR] startTransparent COLOR=#000000[/COLOR]:[COLOR=#993300]void[/COLOR][COLOR=#000000]{[/COLOR]
btn1.[COLOR=#000000]alpha[/COLOR] = .[COLOR=#000000]6[/COLOR];

[COLOR=#000000]}[/COLOR]
[/LEFT]

So when it’s in the down state, I get the transparent state and stays there. But how do I get it back to normal when the user clicks on another button? I’m stumped on that part of the script.

Any help would be greatly appreciated.