colour given in instruction is wrong if i change colour am i breaking the rules in out of the box competition for nationals

No not at all, you can use any paint colors you wish. "Out of Box" you can't use decals other than whats found in the box, no Resin or Photo-Etch. You may drill hole and fix missing detail lost during sanding.

Read your show flyer or look at the website as some shows allow you to do more in this category:

1) Alternate markings/decals
2) use of tape or paper for seatbelts.

Happy Modeling,
Joe