One thing I have found looking at vintage Airstreams is that the blue seems to fade to any number of different hues. Touching up a spot on existing paint will certainly involve some mixing.
While many feel that Rustoleum "Royal Blue" is a good match for the original color, it seemed too deep of a shade for the GT. I've been using Rustoleum Painters Select "Spa Blue" to repaint the roof vents this year. It's darker than the faded paint but appears to my eye
be a good match for some of the other trim on the coach.
Good Luck , let us know what you decide,