dunno if it'll speed things along a bit but aseprite will create a palette from an image you load and then let you save it out to png or whatever. iirc photoshop can do this too. (example attached) i imagine you could use that to filter out the final result as well
There's a lot of things online that can generate a simple palette from an image, but the one in affinity photo can grab up to 256 colors from it. I assume photoshop has similar or probably more featured than that. The color gradient which gives a gradient both with value and saturation is found if you google color picker:…