Change Color Only If Value Has Not Already Been Entered

I'm trying to create a spreadsheet with columns a, b, c, and d. "a" always has a value and I want columns c and d to change to a's color when they are given values. if "b" has a value when the data for c and d is entered, i want them to take b's color instead. The part I don't know how to do is that if i decide to give "b" a value, I don't want "c" or "d" to change colors if they already have values. e.g. if only a has a value when i give one to c, c will take a's color, but then if i give b a value then d, i want d to take b's color and c to retain a's color.