The GTK color selector is similar to Gimp's standard color selector. The main difference is that you can't select which color space that you want to work with in. The left Color bar is always working in value mode and you will alter the hue and saturation by dragging your mouse in the circle.

An other difference is that the RGB values are represented in percent instead of values from 0 to 255. 100% (1.0) is 255 and 0% (0.0) is 0, depending how you work with colors this maybe a advantage or just cumbersome. The fact is that you should see it as the amount of e.g Red that you want to mix into your color.

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