They have new releases more often than that, for example even within a season, they may have 2 or so rounds of releasing, say, f/w colours. Not all sizes come in all colours, though, it can be hard to predict... As for a metallic grey/dark silver, it sounds like steel metallic might be right up your alley -- they've released it a few times previously, I think? Steel isn't a staple colour, though, so you might either need to consider preloved, or look out if they might re-release it. Magnet is indeed a staple colour, as far as I've seen.
Edit to add: They do release other metallic colours from time to time, it's possible they might have some other grey/dark silver than steel too! So keep a lookout if that's the kind of colour you're really keen on, IMHO...