I've read where some people have hand painted buttercream to get the look of airbrushing.  I don't own an airbrush nor do I know how to use one.  How can you get the same look by painting on buttercream.  I never knew you could do that.  Also, can you get the same look by painting on fondant?  Thanks for any help.

Personally I find painting on b'cream difficult :( 

But painting on fondant is a whole different ball game :)  You use past colors thinned with something like Vodka.  The vodka evaporates quickly so you will have to keep adding a few more drops every few minutes as long as you paint.  As I remember, the finished painting will never actually dry so you will have to be very careful handling the finished cake as touching any of the paint will smear.

How  about using Wilton's color mist?  It is basically an airbursh in a can :)  It's just like spray painting anything.

You can mix buttercream with food color then thin it with corn syrup to make something similar to oil paints. If you use royal icing on fondant it would work more like acrylics. Or you can put some buttercream on the cake then spin it with a bench scraper or a spatula to smear it so that it looks like it's airbrushed on in a smeary manner. That's always been a good way to do clouds or variations in the colors in areas that are supposed to be sky, water, etc.

You can also ice a cake with a thick layer of buttercream, then just use airbrush food colors to paint on it. That's how I did this one: http://acaketorememberva.blogspot.com/2012/10/a-starry-night-cake.html

Painting on buttercream isn't hard if you look at it like you're using paints. I use those foam brushes that you get in the decoupage section of the craft store to use with icing paint because they won't drop paintbrush hairs into the icing on the cake, and you can cut them to be narrower if you need a smaller tip on the brush. 

Here are a couple more articles, one is about painting on fondant but the tips are similar. The other used a painted buttercream transfer, which might be easier for you if you're not comfortable painting the whole thing freehand.

http://acaketorememberva.blogspot.com/2013/05/tips-for-painting-on-cakes.html

http://acaketorememberva.blogspot.com/2014/05/how-to-do-portrait-using-painted.html

Thanks for the help, Kakeladi.

Thank you, Costumeczar.  Very helpful.