What are the preferred ways to fill areas with pencil colors in 2d drawings?
I am the OP of this thread. Following is what I tried again:

Here the color pencils are same, but the drawing sheet is different (145 GSM) and the technique is different.

This time instead of drawing any kind of back and forth line, I started drawing extremely small circles, joined together and overlapping each other tightly, with the color pencils. These circles were drawn like the text of cursive handwriting where we do not pick up hand from the paper till the word is complete and all the alphabets are joined to each other.

This was done with an extreme light hand. Virtually zero pressure applied.
Then I repeated the procedure on the same area with the same color 4 times.

5th time, I applied pressure, and the 6th time I applied heavier pressure.

Basically, the following smooth coloring is the result of 6 layers. 

I have taken this photo with a macro lens so you can see the tooth of paper also. The light in the room was dim, hence the dark photo, unfortunately.

Though the colors are still the same student grade, but the results are different and stunning. I believe if I use a high quality paper with better tooth, I will be able to apply more color which may result in darker coloring.