To my knowledge "natural brown" is not available on the CGT......you may have seen Ascot Brown which is a very nice but deeper brown than "natural brown" for the 997 & 987 models. Natural brown looks great with the blues, red, and a few other colors.....but I have never thought a brown interior looks great in a grey car....just my opinion.
If you like the look of Ascot brown you would probably really like the "Cocoa" leather that is available in the 997/987 line. A nice deep brown with a "marble" effect in it. Great color.
I have seen Terracotta more than a few times.....and it IS bright....too bright for some, but still a great color in the right car.....you should see it in person before ordering it though.
I'm really waiting on more black/terracotta pictures....I think the mixture of the black inside would tone down the terracotta just enough to make it perfect. On a black car it should look stunning.
This is natural brown in a previous generation Boxster....the color is pretty much the same today....