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.
Then treat yourself to a refresher on color palettes, remind yourself about the color wheel and how certain colors work better together than others. Even take a look at how different colors can evoke different moods and feelings when used in a coloring page. When you have these basic skills down, you will be producing good looking coloring pages. From here you can start to think about more advanced techniques like shading, burnishing and blending colors and begin to work with some of the tools that the coloring professionals use.