I think it depends on the vintage.
An early (50-60's) 911 looks good in Steve McQueen slate grey. Maybe with burnt sienna/browinsh leather interior if you're all Hollywood. My indy has a long nose 912 restoration in the shop in this slate grey color. With a patina'd leather Momo steering wheel if would be pretty awesome. The car was beat up tube socks white when it came in. The owner must have tripled his money by now.
For 80's Porsches I like signal green or white. Or silver with Martini Racing stripes.
The water-cooled Porsches 2000's look good in Zanzibar red, yellow and that mettalic orange Golanus posted. Just don't put a duck tail on those big cars.
But this would be by far the most fun
parked next to this when its too rainy for the top down.
http://i648.photobucket.com/albums/uu202/christian900se/Cars%20and%20Coffee/Cars%20and%20Coffee%207-30-11/DSC_0864_2820p.jpg