I think you're right.
That tube on the right looked much different than it did even this morning. If it was dirt around the right side tube, it'd have been there this morning. Wasn't the clouds, because they came and went, and the tube was still colored all day. Wasn't a "reflection from the tress" - as someone suggested, unless they planted some trees around the PBR mid-morning today.
The first color we saw in the tubes at SJC was algae - not a solution of any kind. I think we've inoculated.