Not necessarily, The base yellow could be painted with a brush, and I painted the Catwhisker with a brush. The Up armor yellow is the only one on the knees and shoulders that has to be airbrushed.
But I would recomend Air brushing is All. It's really alot easier and looks better. If you don't have an airbrush, you could buy the cheap kind with the canister of propelant.
Would work fine. If you've never airbrushed before, don't worry. Your painting weatherd knee armor,
not a portrait
It's pretty easy. The polly scale paints need little thinning, and I would recommend the polly scale airbrush thinner. The bottles say you can use Distilled water and that is fine, but I've been airbrushing for around 20yrs now and have discovered that thinners offered by the paints manufacturer work alot better with there product than anything else.
Hope I'm not Rambling here.
Hope this helps. Feel free to ask me anymore questions.
I'm always Happy to help. 
Thanks,
Robert E.