The longer sections of the bars are what I aim for, but they don't always work, so sometimes I have to use other shades and mess with stuff like opacity and saturation.
Whether it be little hints of these colors (like highlights or specks) or most of the drawing, I almost always try to include 'em.
If you want to be more colorful with your art, I suggest thinking of some colors you like most and try to find ways to work them into your art! Find ways to make them work, even if you have to change the other tones of the drawing.
This should be an assignment actually. THIS IS YOUR HOMEWORK. DO IT NOW.