grammatical number - Noun following a list of like items - plural or singular?

If you have a list of descriptors (probably names or colours, but could be other things) which refer to items of the same type, followed by a noun which applies to all of the items in the list, should that noun be plural or singular?

For example:

"If you choose the red, yellow or blue door/doors, then you win a prize."

"If you operate the 'On' or 'Activate' switch/switches, the machine will operate normally."

If neither is wrong, but it depends on the context, what does the plural/singular imply about the items in the list? For example if there is only 1 each of the red, yellow and blue doors (vs if there are 2+ of each colour) does that affect which noun should be used?

My inclination was that the noun should be pluralised as it refers to multiple items (there are multiple doors, multiple switches) however I can see the argument that if there is only one blue door, one yellow door, etc. then 'door' should remain singular.

Thanks!