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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

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.

[attachment=3868]

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.