Removal of henna color from hair is possible but it can be long and expensive process.
The trick is to use the right kind of stripper because using the wrong type of strippers can do really bad things to your henna colored hair.
Your hairdresser should use a fruit-based color stripper (not a chemical one). Only licensed hairdressers can buy this stripper. You need to do a strand test to see if it will hurt your hair.
The first month, strip and then correctively color. Also, you need to condition with a deep heat-activated conditioner for one hour.
The second month and you get another stripping, conditioning and corrective color. After the third month of removing, you will be back to your natural color.
This process would be tough on your hair, which is why it must be done over the course of three months.
It is important to note that this process of removing henna hair dye can be done only if you have healthy hair. If you have damaged hair, this route may not be good option for you.
After several trips to the hairdresser, you might end up spending around $400.
Another way of removing henna hair coloring is by using warm mineral oils. To be honest, they may not remove all of the henna color, but enough will be removed to chemically-dye your hair another color.