Both versions are acceptable. The difference is "gray" is more common in the U.S. while "grey" is the British version (and all other English-speaking former countries of the British Empire). Since British English is the version taught in German schools, it makes sense Porsche would use the British version.
And, BTW, if you use the British version, you should indicate that the
colour is grey, while in the U.S. the
color is gray.
DBear