The letter of the law is one thing - the recommendations of HR lawyers to employers wanting to stay out of trouble are far more conservative. The advice I've heard is basically "don't do anything to create or discriminate against a 'protected class'". So even if it's not strictly against the law to decline to hire or promote people who wear blue shoes on Tuesdays, it's probably a bad idea to make such a policy without a strong, well-documented, objective basis for it.