Key and lock
A key is an item that Chip can use to open a lock (or door) of the same color. Each key can only be used to open one lock after which it disappears, with two exceptions: the green key as Chip in CC1 and CC2 and the yellow key as Melinda in CC2 can be used an infinite number of times.
Locks
A lock on top of a clone machine will refuse to open even if Chip has the required key, as with all other tiles on top of a clone machine. If a block is on top of a lock and Chip pushes on the block without the correct key (as with any other Chip-nonremovable tile), the block is flicked just as with any other Chip-acting wall under it. If the key is in Chip's possession, the block is pushed just as if it were a normal block with nothing underneath it.
An interesting behavior is when a transparent tile is located on top of a lock. While Chip cannot step on it without the proper key, stepping on the lock itself does not use the key. In the case when Chip is killed from stepping on a monster in this manner, the lock will disappear although the key was never used. (Since a lock is not transparent, the Transparency Glitch does not occur here.) However, Chip is unable to step on another Chip on top of a lock, just as if it was a normal non-moving Chip tile.
Keys
Per the Microsoft ruleset, the key is a transparent acting floor tile, and monsters will not "collect" the key. In Lynx and CC2, however, keys work differently: the blue key is erased by monsters and blocks, and the yellow and green keys are monster-acting walls. In all rulesets, a lock remains an acting wall as long as Chip or Melinda does not have a key of the same color; if Chip has a key of the same color, the lock will then be acting dirt.
Note that more than one key of the same color may be collected, which can then be used to open that number of locks. In the Microsoft edition black and white mode, keys will have 1-4 dots to the right of them instead of using colors.
Blue key
The blue key is transparent on the Microsoft ruleset, and can be picked up only by Chip or Melinda, who can use it to open one blue lock. In the Lynx version, in addition to Chip, blocks and monsters will erase blue keys after stepping on their tile. In CC2, the same holds true, but ice blocks and directional blocks do not erase blue keys, and monsters erasing blue keys are treated as collecting them, thus becoming unable to pass no signs with blue keys on them.
If a dirt block collects blue keys and transmogrifies into an ice block, it can then open some amount of blue locks depending on how many blue keys it collected.
The blue key and lock is represented by one dot in the Microsoft Black and White edition.
Red key
The red key is transparent on the Microsoft ruleset, and can only be picked up only by Chip or Melinda, who then can use it to open one red lock. It is the only key in the Lynx version to act the same as its MS counterpart.
In Chip's Challenge 2, Chip, Melinda, and their mirror counterparts are the only entities that can pick up red keys. All other entities will pass right over it, including ghosts and rovers who can normally collect anything.
The red key and lock is represented by two dots in the Microsoft Black and White edition.
Green key
The green key is transparent on the Microsoft ruleset, and can be picked up only by Chip or Melinda. In all versions of the game, Chip can use it to open any number of green locks, while Melinda in CC2 can only use it to open one green lock. In the Lynx version, the green key acts as a wall to other objects.
The green key and lock is represented by three dots in the Microsoft Black and White edition.
Yellow key
The yellow key is transparent on the Microsoft ruleset, and can be picked up only by Chip or Melinda. In all versions of the game, Chip can use it to open one yellow lock, while Melinda in CC2 can only use the yellow key to open any number of yellow locks.
The yellow key and lock is represented by four dots in the Microsoft Black and White edition.