White to black

This is going to bug me.   Last night I was finishing up a pen-and-ink drawing, and needed to touch up one little area.  I rummaged around until I found an old tube of white Pelikan-brand gouache (artist-grade tempera) that dates back about 20 years to my college art classes.   There wasn't much left in the tube, and I could hear it rattling dryly, but I knew it was water-soluble and would probably still be usable. So I used scissors to cut into the tube, tore it open, and dumped out the lump of color within.   But it was black. I rechecked the tube's label. It said, very plainly (in four different languages), that it was white.   I got some of it wet, and it was very black. I smushed some of it back in to paint. It looked for all the world like a nice inky lamp black. I re-read the label again to make sure I wasn't crazy.   So here's what I wonder:   1. Has there ever been an instance of Pelikan plaka changing color from white to black over time? Some sort of chemical reaction, maybe? However, an artist-grade paint that changes from white to black seems like a horrible scandal and extraordinarily unlikely.   2. Could it really be that, years ago, I transferred black gouache to a tube that had held white? I would have had to clean out the white tube to remove traces of white, and then put the black paint in (presumably by squeezing from one tube into the other through the openings). And then I would have neglected to re-label it.  I would then have to forget ever doing this. And why would I have done this in the first place? I can imagine a tube of black getting a hole in it, and me deciding that I need to put it into another container, but why into another tube? This would have been at a time when I had access to other little empty bottles and jars.