Fceumm has a palette called “Default”, could someone transfer this palette to the Nestopia core? I tried several palette generators for nes and transferring them via custom.pal file to Nestopia, but I am not experienced enough to do it correctly (the colors did not match the Fceumm “Default” palette at all).
What’s so special about FCEUM’s “Default”?
There’s a Shader called NES Raw Palette Decoder, why don’t you try that and use the RAW Palette in your emulators. As far as I know that’s the way to get the most accurate NES Colours.
I personally use the Sony CXA2025AS Palette which is available in all the NES Cores I’ve tried.
I used Sony CXA2025AS Palette for a long time, but personally the Default palette in Fceumm seems better to me. The colors in this palette are much more “natural”. Almost all of these palettes have much darker colors than they should have. Because of that red looks like brown, pink like purple, and purple like blue 
https://www.mediafire.com/file/tedwobervce7rcg/custom.pal/file
See if this works. I just copied the palette data from this file in FCEUmm and wrote a simple script to convert it into a .pal file.
About palette generation, I’ve worked on my own NES palette generator, but it’s not completely finished. Find it in shaders_slang/misc/patchy-color. The palette generation in nes-color-decoder is a little bit questionable in my opinion.
Just because a palette is included in an emulator, that doesn’t mean you’re able to generate it in a palette generator. Every palette or palette generator you come across can have different opinions about how to deal with the NES’s out-of-bounds colors. Preset palettes especially are able to do basically anything they want.
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