Middle mouse button on Color Palette? (Indexed mode)

I’m trying to figure out what does the function of middle-click to the color palette in Indexed mode?
It’s just randomly spawning a block of color around my pixel art.
Anyone can explain it?

Thanks!!
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Indexed mode requires one color to be designated as “transparency”, which is usually a 0,0,0 black in index-0. Middle click selects which color will be the transparency, so when you pick another one it’ll turn the current transparency in the image into a color instead.

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