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!