Take your time. The only rule you have is to relax, lose yourself in the page and lay down some color. A coloring page is a marathon, not a sprint! Coloring is a great activity to fit in_between other jobs in your day _ waiting for the kids from school? Spend 20 minutes coloring. Got the house to yourself for 30 minutes before your friends drop by? Start that new coloring page!
If you finish coloring and find yourself being critical of the end result, maybe a few tutorials on color control and shading are all you need! Or, if you find yourself getting bored with the same old colored pencils or markers you always color with, a different set of materials might be all you need.