Any rarity can go in your deck, but only Blue can ever be placed directly onto the board. A higher-tier pick becomes a "blueprint" — merging 2-4 Blues (or a mix of tiers, including already-fused ones) requires a matching blueprint still sitting unused in your deck, and consumes it to flavor the result. You can fuse straight from Blue to Orange in one go if you have the cards for it and an Orange blueprint. No blueprint for a tier in your deck means you can never merge into that tier, so pick some Green/Red/Orange cards if you want to fuse up. Everything you pick is visible and available from round 1 - there's no draw step.