Valve caps must be used as they provide an important secondary air seal and prevent dust and foreign matter entering the core of the valve.
Coloured caps (either Red or Green) indicate when a tyre is inflated with Nitrogen gas rather than compressed air. It is not advisable to top up your tyres with compressed air when they are inflated with Nitrogen.
Tyre valves are an integral component of a tyre's performance
So when it comes time for replacement, for safety reasons the existing valve(s) must be substituted with a new valve(s) of the correct type.
The value core provides important protection against air loss and should be substituted periodically to ensure optimumn performance, as rubber components within the valve mechanism will age and could compromise inflation pressure