Make a digital palette. That's just a small file where you paint your initial colours and colour pick from them so that you don't gradually desaturate while working on your image.
Limit your colours. Using a small number of pre-picked colours beforehand can ensure that your image has a better colour range when finished.
Try to understand colours, checking out other people's work is the most direct approach but a little research into colour wheels and colour catagories will help.
Practice, get to understand your software, one advantage of digital is that you won't waste paper so mistakes or inspirational moments won't waste materials.
By the way, airbrush emulation is considered a little old fashioned. In 2D, natural media effects is where it's at. The airbrush tool is very useful but again, if you do some research, you'll discover that more accomplished individuals (I don't mean me) use it as an additional brush rather than a main brush.