There are lots of marine plastic valves throughout the world. There is a name something like "marulan" that I have heard of, but I think it is a material, not the manufacturer.
The point is that plastic as such is not a bad thing for the sea cock. OK, Blakes etc have some fabulous gear
, but hugely expensive, heavy and still subject to dezincification if we get any electrolysis
There is something about the EU prohibiting plastic skin fittings (through-hulls in USA) but I do not know the rules as to where and why.
Important thing is to limit UV exposure for most plastics, especially the PVC ones. PE in all its grades serves well and the newer Glass-Filled Plastics are fabulous.
Often, the colour of the handle is a guide to the intent of the valve, in terms of the fluid - Yellow for gas; Brown for oil
; Blue for air etc - all part of the International colour coding systems. I think Red is the "general service" type of colour.
So..... Many plastics are great, do not dismiss them.
Some commercial survey
licensing systems prohibit plastics, under certain conditions - but some do not!
Colour means little other than someone's intent to indicate something.
If you know the year of manufacture of the boat, ask the manufacturer what it is and where to source replacements