Card Color Pies/Faction system

Hey everyone! I've been developing a TCG over the past 4 years. It's cleared a LOT of hurdles, resource system is solid, card cost, abilities, everything works great and playtesting is fun but I've run into my final design problem and I was hoping for some ideas from the community!

The game is a time themed creature battler. The player pulls events, Antiques, and entities from all different points of history to do battle for them. Each deck in 42 cards, 40 cards in your main deck, 1 location card and 1 "Historic Entity" that helms your deck and is what your decks game play is generally built around.

I've run into the "Yugioh" dilemma. There is no color system to separate cards, there are no factions or clans or houses so in playtesting we have just ended up with "The Deck" aka a pile of all the good stuff.

Does anyone have any ideas of how I could create different "colors" of decks? My original thought was doing cards by Era, and your "Histrotic Entity" would determine the types you could use i.e.

1) Pre-historic 2) Ancient 3) Modern 4) Future

But I don't like the way it sounds and it feels clunky in my head. Any feedback or ideas are appreciated! Thanks for you time in reading this post.