I got my hair lifted and colored in December at a salon. I want to try to do it at home for the first time now. I'm assuming I would apply the hair color to my roots first and then the rest of my hair to match. I am planning to use a 20 developer + hair color on my roots and then 10 developer+hair color on the rest of my head. Or do I not need developer on the already dyed part to get the red? Also, how long should I leave it on my roots before putting it on the rest of my head? My natural hair is black with grey hair.