the protected color is in the rulebook. states that yellow is protected since its the color of the paint used.
on a side note: it used to be orange and/or yellow a few years ago because they kept switching back n forth between fills or had both lol. ive heard some people say orange is still protected but you should be fine with an orange ego.
nppl rules will allow orange (as stated yellow is the protected color).
word of warning orange was at one point protected, as was pink. They seem to change it up & it is possible orange may one day again become a protected color.