This SpaceX tweet (found in CNET's SpaceX camera captures incredible view of rocket part returning to Earth) shows a video taken by a camera in one half of the fairing on the recent STP-2 mission and it says:
View from the fairing during the STP-2 mission; when the fairing returns to Earth, friction heats up particles in the atmosphere, which appear bright blue in the video
I made a montage of four screen shots near the end of the video, where a purple donut appears. Combined with the patterns behind it it tends to make what looks like a peace sign.
What causes this purple donut?
update: Here's the new 05-July-2019 Scott Manley video. He sidesteps the donut, but he does mention that he suspects the IR filter is removed from SpaceX color engineering cameras, so near-infrared (but not thermal IR) results in a purplish color.