What colors of thread should I use?

Three Thread Colors to match any shade of fabric:

Does anyone here really follow this? How does it work for you?

You really need to figure out what works for you.

Many folks serge with one color of thread all the time.

Some folks match all 4 cones to the color of the fabric, some folks only choose to match one needle thread color to the fabric.

The photo suggestion below misleads new serger users to think they are really not going to see the seam construction threads.

When a serged seam is stressed in the crotch area or side seams if a snug fit - the stitching will always show unless the are has been specifically reinforced.

Read up on this here