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I've seen various custom / coloured gauge face sets for sale for the 991 but have no idea as to how easy it would be to swap them out. I guess it depends upon how handy / comfortable you are with a bit of DIY.
A quick Google took me to a video by AutoAmateur who did this on his 991 a while back:
I quite like some of the coloured gauge faces and yellow looks pretty neat, especially if there's other yellow on the car like belts & calipers etc.