The color molecules in red hair color are large, meaning that they can escape easily, thus resulting in fading hair. In order to preserve this you have to keep the cuticle of your hair closed. Shampooing your hair aggravates the cuticle causing it to lift and thus the red color molecules will escape. Also, using hot water will also cause the cuticle to lift. Your best bet is to definitely not shampoo your hair for 48 hours minimum after having your hair colored and then to shampoo it in the coldest water you can tolerate. Also go as long as you can in between shampoos.
Henna hair color is great, but you also cannot get it out of your hair. So this means that if you use henna hair color but decide that you want to get rid of it or do something different, you have to let it all grow out, before you color it. If you try to color hair that has henna color on it, it will not lift so you're pretty much stuck with it. Unless you want to cut it all off.