Can anyone demonstrate how this rollover is done using actionscript?

Hi everyone,
does anyone know,how the rollover on this site http://www.braingiants.com/v5/welcome.html buttons can be accomplished using actionscript. I like the way it the colour box behind the btn changes on rollover and goes back slowly on rollout.
If anyone can point me to a tutorial or teach us how to do this, it would be great!
thanks,
Marjorie