I've only henna'd twice and I co-wash right after and didn't notice any fading at all. But I bet it might have something to do with what conditioner you co-wash with, and some people's hair just fades more easily than others.
2c/3a modified CG
HG product: AG Recoil... Searching for others...
HG method: pixiecurl method of drying. I'll never go back!