I'm traveling right now and didn't bring a copy of the Concours Guidelines with me (can you believe it?), but you lose X points for painting a Healey an original Healey color that was not offered for the particular model of your Healey (e.g., Golden Beige Metallic on a 100-Six), and you lose X + Y points for painting it a color that was never offered on any big Healey.
It's a funny thing though, in all my years judging cars, I don't remember even a single one that was not an original, appropriate color being submitted for judging. It just doesn't happen, or at least it hasn't yet, to my knowledge.
This is probably due in large part to a widespread misimpression that painting the car a color other than its original color disqualifies it from concours judging. Unfortunately, despite years of publicity and providing information and other attempts at educating folks on the subject, there are still many, many people with wildly inaccurate ideas about concours, and also unfortunately they are quick to condemn it. Really, it almost seems at times that some people actually want to find reasons to dislike the whole subject, so they make up things - that are not true - about concours to use as justification to condemn it.
Anyway, the bottom line is this: Even if your Healey is painted some color that was never originally offered on any Healey, it can still achieve Concours Certification.