I think he means the item key because I'm pretty sure the other way would take scripting
Not if you have VCE: A yellow key could very well be a blue key in disguise
OnKeyMismatch -> Client VCE If Var -> KeyName = 1
OnVarTrue -> door -> toggle
OnVarFalse -> Client Msg -> "WRONG KEY BITCH"
The only thing you have to watch for is to make sure you set your MOD VAR for the key.
Place a brick down, wrench it
OnActivate -> Client VCEMODVAR -> KeyName SET 1
There you now have a custom key! Also the door has to be the opposite color of the key (which gives you more options opposed to only 4 colors anyways)
Good Luck