Mid_range pencils from brands like Staedtler or Faber Castel offer softer application, which allows you to get more color into the paper. This can produce a more vibrant color and reduces the chance of white flecks of paper showing through. Softer wax pencils are also better for shading and blending.
Overlap Colors to Create a Blended Look. I like to lightly color the dominate color first and leave a bit of paper exposed for the wax of the second color to cling to. In the liver I colored (below), I started by coloring a layer of red, then I colored some brown on top. Then I colored more red over the top. I find this kind of layering to be really fun!