The colour you have there is not bad but I would suggest maybe another coat?
Just to share my experiences with the humbrol paints:
For the humbrol paints such as the silver they do tend to come up better with a grey primer. I now use Vallejo primer as it comes up better than any of the others I have tried. Vallejo paints are a joy to use in an airbrush.
I used the humbrol 11 with and without primer and saw a marked difference. With primer the colour was more vibrant and picked up better. Without primer the colour was darker and flatter.
How much you stir the paint and thin it can have an effect. The paints have variances from tin to tin. Example for yellow I had to do roughly 5 mil of thinner to a can. Orange was more like 10 mil because it was so thick.
Lastly, let your layers dry for at least a week before applying more.
Hope this helps. 