With focus on the use of the language, what specific trait do the following three three entries have in common?

gray airplane
favorite neighbor
colorful jewelry

Here is the SOLUTION.

THE SOLUTION:

All the words are spelled using American English.

In British English,

GRAY AIRPLANE would be GREY AEROPLANE;
FAVORITE NEIGHBOR would be FAVOURITE NEIGHBOUR; and
COLORFUL JEWELRY would be COLOURFUL JEWELLERY.