Coloring
Coloring digitally is the most unique part of digital art. Unlike traditional art, digital art allows you to work in layers with filters, opacity, and all sorts of other options. This allows you to make all sorts of amazing effects (with some time and effort). I have been making digital art for around 3 months so I'm still relatively early in my career, which is why I tend to stick to a more "cel-shading." This lets me pull of vibrant illustrations without having to dig too deep into the painting tools available to me (yet).
Making sure the illustrations color is balanced is important, as you don't want certain colors to be too heavy or for details to pull too much attention. This requires a basic understanding of color theory, and the more minute details come with time and practice.