I replaced all my windows with grey ones – it was what held up my whole renovation job by a couple of months!
I looked at plastic originally, but the only company that makes the frame material without a woodgrain effect (in grey) is Rehau. My local seller, with the best reputation, wouldn’t sell me any because even though there’s a 10 year guarantee on them, there’s only 5 years on the colour-fastness.
I was then going to go for aluminium, but again, the places that sold them said I’d be a idiot to pay a premium for them when the cheaper options were better (insulated, at least).
I was then going to get aluminium-clad wood, but the leadtime was too long as I’d already started knocking my house to bits.
I ended up with Accoya wood (painted with ral7015), which is supposed to last for 50 years… I’ll have to wait and see!
Edit – the best option is alu-clad wood made by the likes of Rationel and Nordan. I’m happy with what I’ve got, but of course they are going to needed painted once in a while.