I should have qualified this statement. All I've seen and read indicates the chassis was painted the same color as the body panels; however, there's always the caveat that the factory people 'made do' with whatever parts and materials they had on hand if necessary. Also, have heard there was variation in the shades of paint--the formulating possibly wasn't as precise as modern paints--so I was speculating the chassis could have been painted with one batch of paint and the body panels another, and there could have been variations in color due to blend, moisture, humidity, temperature, etc. It's possible--this is speculation too--that the painter who shot your chassis didn't bother to clean his paint gun too well after painting another chassis (maybe a BRG one?). If you have the BMIHT cert for your car and it confirms original color of Healey Blue then my theory may well be correct.
Note 'Healey Blue' seemed to have changed over the years; usually it's a silvery, semi-metallic blue, but I've seen a non-metallic almost turquoise blue called Healey Blue. Florida Green--I've only seen it in pictures--is a light, almost turquoise green