How to draw bubbles with colored pencils

From the coloring book Atlantis by White Star Publishers

Someone asked me how I did the bubbles and I said I erased the outline with a Koh-I-Noor pencil eraser after I've laid down the base color on the page. Someone made a comment and said one could also use a burnisher to do that prior to coloring on the page. The burnisher prevents the colored pigments from being absorbed by that part of the paper. I knew about that before and was glad I was reminded of that technique.

Anyhow, after doing the second and third layer the background now becomes darker. It's the right time to shade the inside of the bubbles since the color inside should match its surroundings.

The way to color is to move your strokes around following the spherical shape of the object. Also introduce darker shades of blue to add dimension. 

I didn't draw the ones with the black outline, that came with the image. I did the white bubble though.

The interior of the bubble should show the other side.

Finally I erased some lines to show the highlights. Did I already mention that I traced all the white outlines (including the highlights) with a white pencil (Luminance)? Yes I did, to create some opacity.

Here's a time lapse video of me doing three bubbles. Notice I left the outlines white. http://youtu.be/5yS-QC7j7X4