Doing realism is for a start all about looking and trying to coppy a reference as exactly as possible. When I say coppy I mean study your reference. You want to learn how light affects shapes and forms and how the difference between lighter and dark areas create those forms and shapes. You'll notice that after a while you start to recognise certain things that make something look realistic and than you can start to "tweak/adjust" your references.
Don't just apply paint, know what you are doing and trying to achive. If you don't know what the outcome of what you are about to do is don't do it (and paint a test piece). Before applying paint ask yourself.
-What am I seeing on the reference.
-What makes it look like that (how does the light affect shapes and forms)
-What texture does it have
-What method do I have to use to get what I see on paper
-Am I sure that what I'm about to do with the airbrush will get the result I want
Don't start with color stuf. Realism at the start is hard enough as it is without the added difficulty of having to do it in color. Also don't start doing color with out having a more than rudementary knowledge of color theory. If you want to do realism in color you will have to read up on it (you can't fix a car if you don't know how an angine works).
For most methods used to achieve realism (there are few "schools") the base is still an initial layout of your light and dark areas so doing a few in black and white and knowing how it works is pretty essential.
There are some excelent step by steps and tutorials on this forum it might be a good idea to have a look at those. One thing I always start with is greyscales, I did a small aricle on that which is here:
http://www.airbrushforum.org/threads/greyscales-the-basics.9901/
A step by step that uses that theory for a black and white portrait (keep in mind that this is how I do a prortrait, it's not the best and not the only method)
http://www.airbrushforum.org/threads/black-and-white-portrait-skin-texture.14178/
In regard to the paintings you posted.
I don't know what size they are so it's hard to say if you can do the detail requiered. Realism doe requier a good control of you airbrush though so if you notice you can't manage them it's back to dot's and lines. There is no way around it you'llhave to learn to walk before you can run.
In the paintings you posted the transitions are a bit harsh. To get natural transitions you'll need to build your transitions slowly and in a lot of layers . If you want something lghter don't spray a lot of opaque white, reduce your whitetill you can hardly see you apply it and than add layer upon layer till you have the desired gradation. This gives a lot more control and is a lot less prone to error. This also means it will probably take 10x as much work but realism takes time, there are no shortcuts.
I mentioned this in the herbie post. Never ever outline, its just not there in nature and instantly kills any realism you have going on.
The above is as always just my humble opinion, and I realise it may be a bit harsh, it's posted to help though not to put anyone down.