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Brainwashing does not necessarily mean convincing your opponent's followers to come over and fight for you. It also means convincing more people to join your side than join their side. In this sense populate is a brainwashing/control (flavour, not gameplay) mechanic; you are convincing more creatures to fight for you. In the context of the Selsnya guild the flavour of populate is not that of a purely green, 'generative' mechanic.

I will not comment the previous statements directly... this would create a very long reply.

What's the Easiest Thing About This Color Pairing?

The easiest part about this color combination comes from the fact that R&D has a wicked understanding of these two colors. Often, white becomes more a 'yellow' or 'orange' ... an aggressive color that uses light as a source for heat/fire. On contrast, green doesn't purely focus on aggressive beatdown either.

In the past, green was the color with powerful weenies like Pouncing Jaguar , Wild Dogs , Skyshroud Ridgeback , Albino Troll , Muscle Sliver , but also powerful, combat-oriented heavies like Thorn Elemental , Child of Gaea or Ancient Silverback .

White rarely had creatures with converted mana cost 1 and a high power and it didn't have powerful heavies. Back then, in Urza's Saga era, Master Healer or Elite Archers would have been the 'rare cards' in your booster packs.

With the time passing, Mark and others decided to give white more weenies and green more heavies, but they couldn't decide whether white should be aggressive or defensively. The same is true for green. All in all, right now you have a nonsense mix of all aspects in both colors without a true distinction between each other. Green has Fog , white has Holy Day ... green has Naturalize , white has Disenchant ... they don't share philosophy alone... they even share the same cards.

Wait... there is a difference: Green hates flyers!

You see, R&D has no clue. And that's the reason why most green-white multicolor spells feels like they could have been monocolored as well and are only multicolored to keep the casting cost lower than with monocolored spells. (see Watchwolf vs. Elvish Warrior ).

What's the Mechanical Heart of This Color Pair?

Now, when I talk about the mechanical design of a color pair, I'm referring to what is the one mechanical element that most defines the interrelation between the two colors. For green and white, this is creatures. Both colors are focused on creatures and both colors define much of their philosophy and mechanics around supporting creatures.

We have a different opinion on this matter. Each color focusses on some sort of creatures. I can see a black Zombie deck, a blue Wizard deck or a red Goblin deck as well. Red gives creatures a power-boost and blue gives them some sort of evasion.

So to answer this question in general...

If two colors share a common enemy (color), the "Mechanical Heart of This Color Pair" is the opposite of that color's identity.

Since black is the enemy color to both green and white, you just need to look at what black is all about. If you inverse this... doing the opposite... you've found your "Mechanical Heart".

Black is the color of death, fear, and amorality—these are the twisted values of the darkness.
...
Black mana opposes white mana through the conflicts of darkness vs. light, corruption vs. purity, and the needs of the individual vs. the needs of the many. Black mana opposes green mana through the conflicts of death vs. life, decay vs. growth, and greed vs. abundance.

Beside the nonsense part of black being the "individual" loving color or green being about "abundance", you can still recognize that black is about death, fear, darkness, corruption and decay.

If you inverse that, green and white are about life, hope, light, cleansing and melioration.

life and cleansing are easy parts. And all the others aren't that difficult. The real problem comes from the fact that you have to design a unique ability for the Selesnia guild.

Convoke
Convoke is a wonderful but also highly problematic ability. It is wonderful, because it requires a community to achieve a common goal. However, Convoke was designed poorly, because spells with convoke have a higher casting cost.

Now you had the chance to either cast a 3/3 flying Soldier for 5 mana and have some other goodies (see Shepherd of the Lost or Castle Raptors ) or you can have the same creature with convoke for 6 mana and no goodies.

The result is that you get actually punished for using convoke. It's usually better to use another form of mana acceleration and cast a similar creature without convoke. If you want to cast a Leatherback Baloth on turn 2, just cast a Noble Hierarch on turn 1.

If convoke would not increase the mana cost, you would have been able to transform any creature into Llanowar Elves for that spell, including a Wild Nacatl . This allows you to avoid mana acceleration creatures and stick to beaters. Beside the fact that it would hurt me to tap the Nacatl early in the game to create mana, you would have limited yourself to convoke. With the Llanowar Elves or Wild Growth , you forgo a bit beating power to be able to cast ANY or ALL creatures one turn earlier. With convoke, you are limited to creatures with this ability.

Convoke shines, if you use the hidden "haste" effect (you can tap creatures that have entered the battlefield that turn)... and really shines on instants (see Gather Courage ). Beside the fact that the mentioned spell doesn't really suffer from the "increased mana cost" drawback, convoke allows you to play reactive spells even if you have no untapped lands.

That's the reason why it is a pure white ability whenever I design cards: If you make convoke nearly free of cost, you can balance other things out a bit.

F.e. a Lightning Bolt is always better than a Healing Salve . You can cast the bolt whenever you want. To be able to prevent damage with the salve, you must not spend all your mana, even if there is no reason to cast the salve.... you need to stay prepared. So you simply wasted your mana and lost game speed.

When I design new cards, I use convoke mainly on defensive spells. Since white doesn't have a Day of Judgment in my "universe", it has a Mana Leak with convoke instead.

PopulateThe good news first: It interacts with the "old" Selesnia token mechanic (which was used to make convoke less a pain in the ass) without copying/repeating the old ability.

The bad news: It's poorly designed as well.

Although Mark is right that it's better to avoid card dependencies and create some spells with populate that actually create creature tokens themself, you can't be sure which token will be copied. And to avoid shenanigans, you'll have to suffer in form of an increased mana cost.

Let's say I use a Cackling Counterpart or Mimic Vat to create a token of an  overpowered creature, populate effects could 'ruin' the game balance. Just copy a Knight of the Reliquary each turn.

Conspire
Although this isn't a green/white ability, I add this here to show an alternative approach that combines populate and convoke. Simply design a few spells with conspire that put creature tokens on the battlefield, search for land cards, gives you lifepoints, etc. And you'd also create quite the same flavor. IF you have a community, you get an additional bonus.

Any Final Thoughts?
I would have avoided the token-based story completely. It's the green way to create card advantage and avoid targeted destruction, but I'm still not a fan of it in white.  I would have rather concentrate myself on lands, enchantments, shroud, hexproof and lifelink.

Black is centered around targeted destruction, but also on global -X/-X effects, so a green-white populate strategy feels stupid. Black has graveyard abusal spells, so I could  also imagine a green-white ability that's centered about the opposite.

All in all. It's nice to see the tokens strategy returning, but I'm a bit disappointed that it isn't something better.