usually something completely reckless can lead to a red. When I say reckless I mean like potential to snap someone's leg or shatter a knee cap. Cleats up slides into the legs usually lead to reads. Also fouls that lead to clear goal scoring chances are usually reds. Yellows are just an accumulation of fouls or something a bit too hard. There are guidelines but It's really up to the ref
Yeah, cards are basically a matter of severity and how dangerous/reckless/intentional what you just did is.
To go to a straight red card before giving a yellow card happens, but it's rare, because it's basically like saying, "What you just did was so fucked up, I'm not going to give you a yellow, I'm giving you two yellows, now GTFO."
Generally you're calling for a card on hard tackles, particularly from behind, where it basically looks like going after the ball is secondary. Also there's the yellow you take when somebody has beaten you, and you just grab them and stop them. Better to give them a free kick and take your card than give them a breakaway.
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