I had heard the term CO washing bandied about and always assumed it meant people washed their hair with coconut oil. I'd tried the whole washing hair with coconut oil, and while it served as a great deep conditioning treatment my roots were always left looking a tad greasy, even after washing with shampoo. I also wanted to avoid coconut oil as I had started to find it stripped my colour faster - I literally saw permanent dark dye wash out of my hair for weeks with every coconut oil treatment. I was keen to hang onto my colour longer after my hair colour disaster.
I actually discovered shampoo wasn't necessary by accident. I can't believe for years I had thought Shampoo was essential. I had switched to a natural, sulfate-free shampoo years ago and this had made a huge difference to my frizz-prone tresses. Looking for different ways to preserve my colour longer I read that not using shampoo can help, as this strips the colour with every wash. I was thrilled when using conditioner alone left my hair clean and smoother. It is best if you use a conditioner free of silicon. Although the conditioner I use contains dimethicone which is considered a silicone-based polymer and can lead to buildup of silicon in the hair. What's wrong with buildup of silicon? Well, nothing too serious. Your hair may appear dull, lifeless, and greasy with time. I've found though with my naturally curly hair, if I avoid silicon conditioners my hair tends to frizz more. The silicon works to smooth it down somewhat. You could always just wash with shampoo every now and then when you feel your hair needs a good cleanse. It also depends on what other products you might be using (serums, hairspray, gel, mousse, wax, etc) that can also buildup in the hair.
So far, CO washing has worked really well for my hair which can do with a bit of product helping to weigh it down.
Not only will your hair thank you, you can reduce hair care costs by using shampoo less or not at all.