What are these purple corners in the color swashes?

[Ofnuts]

Sometimes the color swashes in the Toolbox have purple corners:

   

These purple corners indicate  that the selected color is "out of gamut", i.e. that it cannot be accurately represented in the image. This happens:

• in color-indexed images for any color which is not in the color map
  
• in greyscale images, for any color which is not neutral gray
  
• in RGB images, when the color is picked in a non-RGB color model (HSL, for instance) and has no corresponding RGB equivalent. In this case, the color selector uses the same purple color(*) in the selector widgets to indicate the out-of-gamut ranges.
  

(*) this color can be changed in the Preferences

[rich2005]

To add to Ofnuts post.

A result of Gimp 2.10.x defaulting to Lightness - Chroma -Hue (LCh) color space in the color picker. The magenta in the sliders indicating 'out-of-gamut' 

If it really annoys then switch to Hue - Saturation - Value (HSV) color space, Reset or pick a color to lose the indicator.  Unfortunately, no way at the present to set HSV as default.

30 second example on imgur:  https://i.imgur.com/dOulTfH.mp4

Then this sometimes comes up in one form or another. The change color dialog shows patches of (out of gamut) magenta.

   

It means the user has Color Management and Soft proofing turned on. If you are not going to send your masterpiece off to a printing company, go into Edit -> Preferences -> Color Management and set Image display mode to No color management to lose it.