Agreed; this is mainly what I have done. "Gray" is American and "grey" is English, although a proper name or title will use the specific form that it uses. I forgot about the scientific measurement "gray", although that is a good point too.

Although it mentions audience, also note that many things have international audience (such as documentation for protocols).

There is one further consideration though. I generally will use American spellings in computer codes (and Italian for musical notation). I am Canadian though, so I might write a program and the documentation might say, for example, "the COLOR command changes the colour of..."