From my limited view:
If a Greenman attacked a Purpleman because he is a Purpleman. That's Racism, and other criminal acts. Yes?
If a Greenman kills a Purpleman because he is a Purpleman but with no intent to destroy all Purplemen. That's Racism and Murder. Yes?
If a Greenman kills, or plans to kills, many Purplemen because he is a Purpleman. That's Racism and mass murder, or the plotting there of. Yes?
If a Greenman kills or plans to kill every/most Purplemen in a region or the world. That's Geneocide, or the plotting there of. Yes?
If Greenmen attacked Purplemen because the Greenmen were angry because of the situation that the [Purplemen and Greenmen forefathers] put them in and cannot make peace with themselves, thier forefathers, and thier neighbors(Purplemen) then when/if the Purplemen retaliate against the Greenmen attack(s) and possibly 'innocent' Greenmen (hurt/killed) that were anywhere near the Greenmen that probably attacked the Purplemen is that Genocide? No. It means they are fighting and there was possibly 'innocent' Greenmen were hurt/killed.
If in that situation for every four Greenman and there is one Purpleman that is killed in the [Greenman attack and the corrosponding Purpleman retaliation] isn't Peace, even a bitter one, better? Unless the Greenmen are planning to continue no matter the cost, and/or genocide of the Purplemen is really the intent.
But lo!, then the Bluemen come along and say "That is not a fair fight!! The Greenmen used rocks, and sticks, but the Purplemen used swords and wore armor!" So what?
Stop picking fights with the big kid in the schoolyard if he's happy to leave well enough alone, and eyes won't be blackened and noses won't be bloody. Sure, get lucky in the fight once in a while but "WHY?!", especially if the the little kid in the playground is trying to make the big kid get the little kid's kitten out of the tree that the little boy's grand-father scared into the tree in the first place, and then get the big kid to chop down the tree for the little boy so that his kitten can never get stuck in the tree again?
The Greenmen don't have to forgive, they don't have to forget - do the best with the cards they are delt! Perhaps diverting the energies of War to the energies of Peace, Education, and infrastructure would give a better chance at change? Yes? Maybe?
Perhaps the Greenmen know education and infrastructure is EXPENSIVE to make work. It's EXPENSIVE to make a good life starting from scratch; but it has to start some where. But sadly, it's cheaper to go to blame others, and make War with the Purplemen and hope that something magical would happen when it's all over that it'll be better for the Greenmen. In the end, nothing changes.
It really doesn't matter even because everyone has to switch colors for the next fight and start the process over...
1. Greenmen fight Purplemen!
2. Purplemen fight Greenmen!
3. * magic happens!(or doesn't...) *
4. Greymen profit!
5. Goto 1
PS: Vote for me for "World Dictator and Evil Overlord!" I PROMISE it will be a much better and happier World! The firing squads will continue until everyone is Happy! Are you Happy? Yes/No?