When you airbrush should the paint (enamel) be a lighter shade than when brush a painting? I am finding that thsi is the case Have I diluted it too much?

Thinning the paint too much could cause the shades to be off compared with brushing straight from the bottle, though not that much. Maybe you could try to back up a little on the thinning and try again.

Thinning of the paint, whether enamel or otherwise, does not affect the colour of the paint. It only affects the number of coats required to cover the surface being painted. Sometimes a 'mist' or tack coat of overly thinned paint can appear lighter.
Just give it a couple of more coats.....but don't overdo it.

Tread.

Thanks Tread, I forgot to mention the additional coats required due to thinner paint.