hair color questions

i'm going to color my hair on monday. the grays are getting to much for me. do i apply the color at once? or wait put the color at th ends once the color has been on top for awhile? does my question make sense? after i color it how can i give it TLC? that way i have no damage. at least not much damage? i have medium to dark hair brown. i'm going to try raven sunset by hydrience. i was going to mix dark brown with a pinch of alburn. i changed my mine because in the hair color looked to red and my hair picks up a lot of red. the haircolor of raven sunset swatch look like what i wanted. so the dark just wouldn't be flat. you know how the color on the box and swatch don't always come out that color. so will see. a little about my hair care routine. i do mostly CO [conditioner only] sometimes shampoo once in a while. as my leave in right now im using BWC. as deep conditioner i use DOVE INTENSE MOISTURE.