In the meantime you can toggle diplomacy colors, e.g. via the button left below the minimap.

As a player, I'd like to share a few opinions based on personal experience. Battalions are interesting and well suited for large scale battles; however, for smaller border conflicts, they're probably not suitable. The same applies to scouting, and lightning cavalry assaults probably require more flexibility and speed than a tightly packed group of soldiers. Historically, it's also more suitable for Rome, Greek city-states, and some civ around the Mediterranean; I find it less suitable for other civ. Therefore, I think it would be more appropriate as an option rather than the default. I'm new to this forum despite playing this game for 7 years, so I'd like to thank the developers and the community for creating and contributing to this game. I hope you all find more common ground and further improve the game.

The only reason people do this is because you can defend well with CS and use them as the backbone of your economy. CS gather 3 resources better than civilians, so there is little stopping people from just massing them and then gradually switching to champions when they bank on resources, as you've said. Let's see how you'll turtle when forced to expand and make actual military units...or risk the opponent out-booming you with better eco units.

Oh, you're the guy who revived HC, thanks! Looks like I haven't explored this game enough. I was wondering if there's a way to allow multiple players to choose the same color. You probably know how, right? Could you tell me which part I need to modify?

Snowballing happens when there is no diminishing return on something you can accumulate, like champions. Champions are also more resource efficient then their CS counterpart, as well as population efficient. There are no diminishing return with them, quite the opposite. Fast melee champions units have no counters, and can hardly be "outplayed" because they can pick battles they want, therefor securing an advantage you have with them is easy. Fast units contribute more to map control, therefore it is harder to recover if the enemy can just easily find and crush anything you try to rebuild. There is almost no limit to how much strength you can coil up with champions, as you replace workers / CS with champions, your strength can grow despite the population limit. The players that do play on snowballing don't send their CS to battle, they keep them on eco and replace them (even delete them) with fast units, be it champ cav, or cav. That makes me say that CS as workforce don't contribute to snowballing, instead they often provide an opportunity for defenders to keep eco, while the attacker is losing an opportunity cost of moving his army around. Turtling is often done like this : build forward defenses, force the enemy to move his army and waste time on defenses, while you keep doing eco with most of your population; therefore you can catch up with any economic disadvantage you had.