The colour buttons can be programmed and labelled in the EpgSelection.py file. If you want the buttons to be different for just the graphical EPG and remain same for other EPGs then just add an If statement to check if self.type ==EPG_TYPE_GRAPH and add the new button mapping code within that statement (linking each button action to the relevant method). If you need more help then just give me a shout.
If I am home this weekend then I will share my code/file for everyone to use. I have managed to make the following changes:
Red button: popup with extra options such as IMDb serch, EPG serch, add timer/auto timer
Green button: shows recordings (movie list)
Green button long press: shows timer list
Yellow button: -24hrs
Blue button: +24hrs
Ch+: page up
Ch-: page down.