Can I still color?

i have highlighted my hair about four times in the last four years and have done all over semi permanant once. now i am halfway back to my natural color and i hate it (light-med. brown)i want to get rid of my natural color but i am afraid that i have allready colored too much and don't want to damage my hair. my hair is to my shoulders and looks and feels in pretty good shape to me.also, if i were to color my hair would i have to go darker than my natural color in order to avoid damage? (i am pretty fair and i am afraid that would make me look like a ghost)please help thanks!Erin