Yellow Tubing
The least resistance, the yellow tubing would suit easy exercises for light women and rehabilitation work.

Red Tubing
The red tubing is an introductory band for most exercises, but may be challenging for light women on some movements.

Green Tubing
Moderate resistance, green and blue tubing are recommended for the majority of purposes. The green will provide enough resistance for most exercises, but may not be enough for some exercises like bicep curls.

Blue Tubing
Blue tubing is the other most versatile resistance and will provide difficult resistance for most exercises and moderate resistance for others. Recommended for men (and strong women!)

Black Tubing
Black tubing is a high level of resistance, more used for full body exercises like squats and lunges.