This May 4, 2019 SpaceX tweet says
More photos from Falcon 9’s 40th successful launch from Pad 40 in Florida and 39th first stage rocket booster landing to date → http://flickr.com/spacex
and shows two images, including the one below. The image is also shown and discussed in flickr.
Just what is happening here and what does this photo show? What exactly causes those relatively pure red and blue regions of this plume?
They remind me of aurora colors.
Click for full size, it's beautiful and something is going on around the engines...