Colour is irrelevant. Algae growth in your tank will change the colour of the rock within a few months or less. After a couple of years of coral growth (if you plan on keeping a reef tank) you won't be able to see very much of your rock anymore period. Liverock is a biological filter and base in which to mount coral. Shape, porousness/porosity(?) should be your main concerns (for a reef tank).
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