In my experience, the term 'woodgrain' is used to refer to mahogany coloured uPVC. If you go to most window suppliers and ask to see their woodgrain windows they'll show you mahogany.
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However, I think this is a remnant from a time when there was either this colour or white rather than a proper technical term. Therefore, while you may have trouble pulling off white (or for that matter, black, green, blue, etc) uPVC with a woodgrain texture, I think it does leave you open to just about any other wood effect colouring.