Why are there flakes in my mouthwash?!

I've never paid much attention to it before, but every time I brush my teeth and then use mouthwash, there are dark colored flakes when I spit out. Like when I use the green Listerine, there were dark green flakes. I always figured the mouthwash had particles in it and never though about it.

I just switched to Crest Prohealth and it's purple, and when I spit that out I have dark purple flakes. I inspected the bottle and the mouthwash is perfectly clear, no flakes in it.

So I tried googling my conundrum like I do most of my daily questions regarding life, but apparently mouth wash is a cure for dandruff (who'da thunk it?) so my google searches don't give me my answers, and so I turn to you!

Have you ever noticed flakes when you spit mouthwash? WHAT ARE THEY? What causes them? My boyfriend thinks they are left over food *gag* but I would think the TOOTHBRUSH would get that out lol... Anyway... Help!