Clairol Foam Question/Hair Dye Questions..?

Ok, so Im thinking of dyeing my hair with the Clairol Foam stuff. I was thinking of using the "Dark Blonde" one, would it look good if i used it on hair that is brightish-blonde with brown/blonde underneath?By this i mean just the top layer is really blondeish, my mom didnt do the last highlights right. >.<.. Would it come out on the box or darker? Also another question is, that the longer you leave on the hair dye the darker it will get? Or is this not true? Because im scared it will get to dark and ill hate it. (This usually happens to me when i dye my hair. >.< )
Thank you !
 
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