I think 420 is something like "weed day", or something pot related at least. i'm also not sure (not a US citizen so...) but colorado might have been one of the first states to legalize weed. So that makes this sign a symbol for pot smokers. Not sure though.
420 is the police code for marijuana people now use it to refer to it though and it is usually used like "420 blaze it" people probably wanted to take the sign to represent their use of weed plz don't question how I know this xD
Theif: Raaaahhhhhhh. *dies*