Why do you say to use offense/defense and not colour? With a change of posession I think the colour is much more definative.
Use whatever works best for the R, but the R shouldn't use 'colors' in his announcement. So, generally speaking, it is best to use what he'll need for his announcement, i.e., offense, defense, kicking team, receiving team, passing team, return team.
E,g.:
"Block in the back, number 27, receiving team, at my flag." (punt or kickoff)
"Illegal formation, more than 4 in the backfield, kicking team." (punt)
"Block below the waist, number 33, return team, at my flag." (during interception or fumble return)
"Facemask, number 57, passing team, at my flag." (during an interception return)
"Pass interference, number 18, defense, at my flag."
"Holding, number 87, offense, at my flag."
Then the R should step through the penalty enforcement, so stay there with him until he has everything he needs, like result of the play, spot of the foul, end of the related run, etc. He may forget the number while he is working through the enforcement, so stick around in front and off to the side of him, ready to feed him the number again during the announcement, if needed. Don't be in a big hurry to whip out your data card to write down the foul - make sure the R has what he needs before you do that. You may think you need to do that immediately, but all you need to record at that moment is the time and player number. You'll remember the rest (period, team, foul, accepted/declined/offset) , and you can fill in that info at the next break in the action.