Some colors are more transparent than others. Some colors are very opaque. Test your colors on a separate piece of paper to see how they work together. Too many layers of crayon can make the colors resist each other. This can make it harder to add more color. Use as few colors as possible to get your end result.
Concentrate on the basics. If you are just starting to color, keep things simple. Forget shading, burnishing, details and texture _ start off by block coloring and concentrating on keeping between the lines. Block coloring is simply the practice of taking your colored pencil and filling an area completely and evenly with that color. Get to know your pencil, feel the weight of it and practice control. Simply filling those spaces with color whilst trying not to go beyond the lines will help you master control and prime you for learning more advanced techniques.