The original front brake oil pot cover on a 675-series bike is a plastic moulding. This replacement is an aluminum alloy fitting, which is the reason it is offered in a colored finish at all: red, black, grey and blue, under both the 675NK and 675SR labels.
On a brake reservoir, the cover is a cap — it sits over the pot, holds the seal face and keeps the fluid where it belongs, so the material you pick changes how it behaves over a season of riding and cleaning rather than how the brake performs.
Metal covers are usually chosen by riders whose plastic original has scuffed, dulled or cracked at the mounting face. An alloy body is the more durable choice in that situation, and it is also the reason the finish can be matched to a colored fairing or bar kit. Two practical points from the listing stay the same whatever colorway you pick: screws are not included — you reuse the original ones — and installation is described as simple, with no instructions required.
The listing states the material as aluminum alloy but does not name a specific surface treatment. If the exact finish process matters to you, ask the seller to confirm it for the colorway you want.