One color models for army men collection

Hello. I just got back into collecting army men/toy soldiers and vehicles for them. I plan on building some vehicles for the collection as well. I'm soon going to receive a Squadron Models 1/72 scale Haunebu II model kit, and I want to paint it red for red toy soldiers. I would like to know what type of brush to use for painting models one color. Should I use airbrush, paintbrush, spray paint, or some other method I have never heard of? Thanks in advance.