Prevent black colored pencil from blending into adjacent colors
Lots of good advice here, but you may also want to consider using a fixative to preserve the drawing as you work it. Fixative, found in convenient to use spray cans although you can get it in liquid form, chemically bonds and protects the pigments applied to the paper. It is nearly a requirement when working in pastels and I regularly use it to protect my graphite pencil drawings. Look for workable spray fix in this case.

So you would draw the dark details, spray fix them, then work the light areas and there will be no smudges. The reverse is possible as well but sometimes the fix will darken light colors. 

Fix is toxic so spray outside or have good ventilation.