It's the same as with any TCG games, Decks are built with either 3 main important things in mind, Damage, Sustain, Cost efficiency

Damage - How much damage you can deal out in a single turn

Sustain - Having an effective balance of both damage but also defence/cost protection

Cost Efficiency - When you use a card form your hand you lost 1 card, It is better to try and use a card but lose no cards, get a card back or at least force your opponent to also lose something on any given turn otherwise you will lost every time to sustain

and that's effectively how TCG goes, This game does a very good job and recapturing what it was like to play TCG growing up in the 1990s

I did in fact grow up in the 1990s and had many many fun duels with friends back then and we would often impose our own rules and stuff to make it more interesting and to have continued fun.

All TCG games are always built around those 3 aspects the only thing that ever changes the game really is effects, spells and anything else like that but I put them into a category called Random because TCG also requires random luck no matter how good you are at it you can't win them all. So you try to build a deck so that you can never draw badly

That's coming from a regional champion by the way, I won the 2003, 2004 and 2005, Yugioh regional championships in Northern Ireland not a prestigious thing really lol but out of 200 other people that's not bad , Plus the free full boxes of cards I got made me some nice money when I sold them all at 18

have a nice day dude

Green is ambush and stun.
Purple is lethal and morph.
Orange is Defender and stat buffs.
Blue is Swarm and multi-summon.

There are other themes and exceptions, but at the end of the day that's the headlining features.

Almost seemed like the originating idea was a pokemon-style system. Purple beats orange, orange beats blue, blue beats purple. But this is not at all some kind of hard-set rule. Just kind of a general concept thing.

As you go through the game you'll gravitate toward some stuff, and can get opportunities to change the rules to buff the stuff you like and debuff other stuff. You will also get stickers to change the alignment of cards... but not a lot of them. You get a fair amount of stat changes, and a few keyword stickers too.

The rest is just my opinion, but I never clicked with purple. Morph is weird and unreliable, and lethal monsters are typically weak in stats. Blue felt strong early game, possibly because I had a good champion, but weaker than my other cards late game. Orange always seemed good, but almost boring? Green is where my final deck main ended up, having an extremely offense-related Champion, and the ambush effect. Don't take that to meant it's the strongest, it's just what I liked and found strong. I think blue still fits my playstyle more, but I never really got a champion that complimented it enough.

Make sure you complete the quest to make your own card, you can make something broken.

I don't know if this goes for every playstyle, but for me neutral turtles were good in any deck. I gave two of them lethal and max attack/hp, and with all my rule changes they were strong stats with defender and lethal for 2 cost. Not super broken, but just solid and useful in many scenarios, and in any deck because they're neutral.

There are a couple of rule changes that vastly change how you play, so don't dedicate too strongly to any one thing. Also due to the ability to change card color, don't completely disregard any full set, even if you don't intend to use them as a deck. You can use them for trades, but think about how they might be used in decks of other color first.