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You can use Aseprite, Paint SHop Pro, Photoshop, to load an image , and save the color palete in .pal format to be used in the tool. 

Or you can create youw own colors using the RGB sliders there.

Thanks for your answer!

I tried saving .pal in photoshop (in cs6 and after putting the pic to 256 indexed color), but pixatool cannot open that specific .pal format. I checked the palette table in /palettes and I see

JASC-PAL
0100
256
0 0 0

I figured 256 is the number of colors and then start the RGB values. Photoshop saves me this:

RIFF  PAL data        (and gibberish so on :))

I will try Aseprite later maybe that will save the correct format.

I still have to say this method is not the best, as much as using the sliders. Of course I'm saying this as one who uses graphic softwares and used to color pickers. I really think a color picker would be useful. :)

Thank you!

I save .Pal files in Aseprite and Paint SHop Pro and they works perfect.

You can test pals form here too: https://lospec.com/palette-list . It allos to download .Pal files.