NFL uniform color

Question
Can you settle a bet with me and a friend?  I was looking online but couldnt find the rule pertaining to the home color jersey.  My friend insists that even though most teams where dark at home, the rules say they should wear white and that most every team uses its option to wear dark at home.  I say the rule is dark at home and a few teams on certain occasions (and the Cowboys religiously) wear white at home.  Please help.

Answer
This question dates back to the pre-turf and even pre-grass era.Once upon a time teams would wear whatever they had be-it dark or white;this went out because of the confusion it caused when throwing a football became popular.Therefore the home team had the choice of what color to wear;which for the most part was the darker uniforms.Why?This was another way to get inside the enemies head to see their white uniforms filthy giving the appearance of a beaten and battered team was thought to have a psychological effect.