red key -> red chest
blue key -> blue chest
just throwing an idea out there... the inventory screen could show you how many of each key you have, and then maybe there could also be rarer skeleton keys that could be used for any color chest...
... yellow key -> green chest, just to screw with people...
You made me lol, but essentially yes. A lil' more complex, like may'be have some "magic chests" that have to opened with say a "mana orb" or something.
Magic Chests are everywhere.
"mana orbs" are rare, only having say 5%> drop rate.
People would then rush to pick these up, as they open more rare chests, which have a CHANCE at higher loot. This would accomplish people having a desire to pick up the keys, forcing them to make a decision on inventory space verse item feasability (or some other word).
Add in various chests like these, like regular chests are common, "locked" chests apear in about 1/4 chests, whereas "magic chests" would spawn only about 1/7 chests. This would cause for there to be enough for there to be a desire, but not enough to where it would be a necessity. That is the only problem that I could foresee with this system.
Q u o t e:
Like someone else here mentioned, you could go the keychain/keyring route, which could negate the idea of generic keys (unless they bothered implementing a keyring just to hold 10 of the same key, at which point you might as well keep 1 in your pocket and be able to re-use it, which would negate the point of locked chests altogether).
With that in mind, there is another mechanic to look at here - special keys could drop and trigger quests once the player right-clicks it or speaks to an NPC about it. The quest could involve anything, of course - escorting, dungeon crawling, go kill X-monster - the reward for which is a map to a randomly-spawned chest that is only visible to the player/party completing the quest and can only be opened with the special key.
I'm also for the blue key - blue chest idea, but I've no idea how you would handle, or rather balance, the generation of both keys and corresponding chests in the same game. I mean, say you found a blue key, but it was known that the game doesn't necessarily spawn a blue chest in your game at the same time the key is dropped. Now you have to hold that key until you come upon a blue chest. Actually, this isn't a bad idea if there is a dedicated keyring that doesn't take up inventory space. That way, a rare key drop becomes as valuable as a rare item, because you'll be hoping to happen upon a matching chest (if the contents are guaranteed to have at least a rare item, for example).
This is the kind of variety that I am talking about. As for the blue key, that is EXACTLY what I was implying with my "magic" chest. Just because you find something doesn't mean your going to use it right away. In my games, I have ALWAYS believed in investing for long-term, and that is what this is. You sacrifice inventory space for a possibility of some epic loot later on. It's not a guarantee, but its enough of an incentive for it to be viable, but not a necessity.