Professional level pencils like Derwent or Prismacolor offer the best quality and typically are the most expensive pencils to buy. The wax in these pencils is softer and the colors are a higher pigment providing brighter colors. The soft application allows blending and shading.
Here are some tips on using crayons for effects in coloring: You can start with a light layer of the color you want to use first. Then you can cover that with darker or lighter color to add life to your drawing, just like the wings on this butterfly.