Thanks everyone.

cobraleader, I'm using Tamiya acrylics and working by the rule that too much thinner is better than too little. I'm usually using 1 part thinner to 1 part paint, but not everytime as occasionally, it looks like it thins easier.

I test sprayed the lighter tone of blue on a plastic drinks bottle, but after that, I just continued to add a bit of blue to the mix and proceed with the darker panels. I wanted the darker blue to be much darker, but not as much as this. I really should've tested it first as it was a big change in tone. I'll make sure to do this from now on.

I've watched quite a few youtube vids about airbrushing, but the more I watch, the more I learn. Some DVDs from professionals would probably be a good thing for me.

Seems like  a good idea to do an under colour, but I can imagine using alot more paint that way. This is the most of the masking done anyway, so I'll just be more careful with the masking tape. It's Tamiya 6mm masking tape I'm using, then I'm using blue Scotch masking tape to cover the areas around it.

I'm not sure if I should touch up the leaked areas with a brush, or if I should airbrush them again? It would only take a couple of minutes to brush them compared with the time it takes to mask them and spray with the airbrush. For such a tiny amount of paint/area, which is best?

Thanks for all the help, folks.