The color varies considerably. The early color was grey. It was near the color of aluminum. The show model and the New Zealand bike were finished in satin chrome and had anodized forks. My bike and Carleton Palmer's bike were built in January of 1950 and had grey paint which I matched to this color:
This is Nason Ful Cryl II Acrylic Enamel, which I used because I wanted an "enamel" look to the paint. It is a Jaguar color. Nason is made by Dupont and is their "non-advertised" paint line that is more reasonably priced, but it is a high-quality paint.
Another color used was a green color. Glyn Johnson used this on his Flash. I can only speculate on this color because it is not on the earlier Flashes that I have seen. I suspect it was first used around June of 1950.
Here is a comparison of the two colors:
The racer I built is done with Poly Urethane paint instead of enamel. I did this only for use on the racer. The handlebar clip in the photo is an original Grey Flash painted at the Factory part from my bike 3407. The brake arm is from Glyn Johnson and reflects his research on his TT racer. I don't have a number for that paint because it was not used on my bike or Carleton's bike.
David