What's the difference between wella colorcharm cream developer 40 o4 20 volumn is used for 4r/356 hair color?

This is a great question I am happy to assist you with. Our Wella ColorCharm Cream Developers are used according to what you are wanting to do with your hair. 20 Volume is usually for deposit and to assist with gray coverage along with the possibility of lifting 1 level of hair color if you are trying to lift the natural virgin hair levels without lightener. 40 Volume is used for lightening natural virgin hair with hair color. 4r= level 4 hair the r=red. Unless you are blonde a 20 volume is perfect for this hair color. If you are a level 7 or higher you would want to fill the hair first before going this dark. I hope this answered your question today. If you have more questions specifically with your hair please call our Licensed Stylists at 1-800-935-5273 to consult with you and find the perfect formulation for your hair and what you have or do not have on it. We are happy to assist you in any way we can. Have a Beautiful Wella Hair Day!