Why is it always a white powder?
Never blue, or green. Heck, you'd think if we were talking spores, we'd be talking brown, or black powder.
And, as Larry has pointed out, we have not yet seen exactly what "tested positive" means. Obviously at least two people received enough spores to infect them, one fatally, but we may be talking about a very small amount of infectious substance being used to maximum terror advantage.