Hi Madrob and firstly a very warm welcome to the forum. You join the ever increasing band of members who have returned to the hobby after a number of years and are now realising just how much has changed.
Your questions regarding paint open up a whole pandora's box of subjects so I would first of all suggest you have a look through our tutorials and hints and tips and read up on what has already been said on the subject.
The first thing that stands out from your post is that Humbrol and Tamiya are two entirely differrent paint systems, Humbrol being enamel and therefore an oil based paint and Tamiya being acrylic and therefore water based. Water could be used to thin down the Tamiya paint but, as it's evaporation rate is very differrent to the base paint the consistancy will vary with time so you are far better off using the correct thinners.
As for matching up Dunkelgelb, I'm not sure yet but I am sure there are plenty who can help. You may find it interesting to read through this thread here though in the meantime:
http://cs.finescale.com/forums/70146/ShowPost.aspx
As for airbrushes there are definately some usefull Tutorials posted on this forum so have a look through them first.
I'm afraid though your Badger is a very basic brush although it should be OK for overall coverings on such things as armour. It will take a bit of practise to get it right. The main thing to think about first is the consistency of the paint. You should be looking for what we consider a milky consistency. That is about the same viscosity and it wets the side of the jar in much the same way. A usefull start point would be 60-40 paint to thinners but you may need 70-30 depending on the paint and the brush itself.
I've just found this which does the trick as well:
http://www.modelairplaneinternationa...s/info/004.htm