So in this case, without multiplexing, and given that you expect the combination of red and amber but not red and green, then for each group of three, you can have one resistor for the amber, and one common resistor in the supply side for red and green.
Now of course, if you use the same resistor value for read and green, the red will have a (significantly) lower forward voltage, and may appear brighter. Since they will not be on simultaneously, you may not notice this.