On other occasions when I have had a bad day at work, I'll want to zone out whilst I color; for these times I often find myself reaching for a mandala. There is something about coloring my way through those circles of pattern that really help my mind to switch off!
When you are coloring, make sure you have a good light source so that you aren't straining your eyes. During the day try and find yourself a nice spot by a window with plenty of natural light, alternatively invest in a daylight LED lamp so that you can have a decent bright light to color under day and night. Some of these lamps even have in_built magnifying glasses which will help you when coloring detailed pages, but will also put less strain on your eyes.