Arts minister, but art thieves. Drugs tsar, but drug dealers. When you put a noun in front of another noun, should it be singular or plural?Sometimes we all need to just look something up, to find out what's recommended or correct! (My blog on Nov. 1 covered rules for using for using plurals.)
If your dictionary lists two (or more) spellings for a word, use the first one unless my editorial style manual (or another style guide) lists a specific exception. If your dictionary provides different spellings in separate entries (gray and grey, for example) use the spelling followed by a full definition (gray). If a dictionary entry is listed as usually or often, use that entry.
If you live in the United States or are writing for publications mostly for U.S. audiences, you should use American word spellings instead of British spellings.