Paper plate with remains of red cabbage so the stain is red in parts (acid) and blue in parts (alkaline).

I know #JanuaryAbstract is over but I'm currently at a church function and we've had meat and potato with pickled red cabbage. This is a used paper plate. Red cabbage is red because it is in acidic vinegar. What alkali is on the plate to turn it blue?
#abstractphoto #abstractphotography #acidalkali