Highlights all over in stead of FULL colour.. WHAT? helppp. :)?

okay, so for prom i want my hair dark brown, matching my dress cause its a coral colour :) im a natural auburn/reddy colour.. so me and my stepmum got some washout hair stuff and its done NOTHING. so my stepmum booked me in for the salon in like.. 3 hours.. and they said a full head colour will do nothing, so shes putting highlights of brown all over... WHAT THE ****? is that right.. hahaa, im scared, i dont want to have loads of brown, then loads of ginger all over :S help.