Arrows and spells used to be the same color, but arrows had to have string or ribbon on them. In practice, this turned to having the ribbon or string wrapped around the stem of the packet instead of trailing behind as intended. In the last rules change, ribbons were removed, and arrows were made blue.I suppose the question is why were colors put forth in the first place and is that reason still a valid one? To me, the only thing I could see is to define what type of cauliflower hand the person was holding so that you could tell if they were casters or alchemists. Also (unless I have the rules wrong), you can steal an alchemy globe and thereby steal the alchemical gas; it would be important to be able to denote what is an alchemy globe in the game.
I don't know if blue is necessary as the bow would be a good indicator that they're probably carrying arrows.