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Is you image color-indexed? If so you have purple corners in the FG/BG color swashes in the tool box, telling you thathe colors are not usable in the picture.
When an image is color-indexed (all GIF, and some PNG) the colors of pixels are encoded as an index in a colormap, and the only color you can use are those in the color map (otherwise Gimp tires to find the nearest color).
You can change your image to RGB using Image>Mode>RGB.
Posts: 5
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Joined: Mar 2021
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Operating system(s): Windows Vista or 7, 8, 10 (64-bit)
Just sharing, because this has drove me nuts for the past two hours. My color picker was always picking a darker shade than what I actually picked.
I have layers in nested Group Layers 4 deep. And one of them was only at 50% Opacity. Setting it to 100 fixed the color problem.