The color of DE does vary, it's not always pure white. There are impurities that will account for the color—minerals and volcanic clay particles. The UltraSorb is much whiter when it is dry, but when you add water the earthen tones really come out. The UltraSorb is baked too at about 600°F from what I've read, to dry and sterilize it, but that doesn't alter the color, and it doesn't alter the amorphous structure of the silica. Calcined DE like what is used for pool filers is heated to a much higher temperature near the melting point which changes the structure of the DE and creates a lot of unhealthy crystalline silica. Never us that stuff.
My main concern was choosing a natural amorphous fresh water DE that had the proper PH for a seed starting medium.
Travis, I have had algae growth on the surface of the DE, but only when I used rain water. It didn't occur when using distilled water. It didn't show up much with the Tomato's though. Luckily the algae is harmless. Fungus won't grow on DE since fungus needs dead plant matter to grow on, which is probably another advantage of the DE—no chance of damping off.
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