Purple colored rim around dark objects, typically on a white background
Purple fringing is the visible effect of lateral color-aberrations.
You can imagine this as is one of the Red / Green / Blue Images on a color sensor were a bit too small, for example the blue image. Instead of reaching the white area , together with green and red, it reaches the black area closer to the image center , where no light were expected at all.
That’s why the effect only occurs where (nearly) white and (nearly) black regions meet. If it’s white in a region anyway, it doesn’t matter is all, say, blue) light rays arrive a bit too close to the center, as the “gaps” are filled with other rays. Where black and white areas meet, however, the gaps can’t be filled.