I love this, I was scouring the 'net for a few days until I found something I wanted. It's sooooooo close to what I want! I am just a hobbiest who enjoys browsing through sprite sheets. Would it be possible to do a '1:1 swap' button? In other words: 

1. Load a sprite sheet PNG with an indexed colour palette with a grey-scale gradient
2. Load a map palette, it also has 16 colours. 
3. Click to map those 16 colours 1:1 from the map palette

Right now I can make it work, but it's just so tedious to click source colour, then click map colour, for 16 colours (32 clicks in total), when I know it's possible you could make it all happen with one click? This would be similar to the 'AUTO MAP' button, but instead of doing CMI match it would just map the pallet to the index. I've enclosed an image to hopefully show you what I mean. I got the result I wanted but it would have been more convenient to just have a button that does it right away, as I already have 16 different palette files ready to go.

Again, thank you SO MUCH for this amazing tool.

Thanks so much for your kind words!  
So glad to hear you like the tool and find it useful.  I've told this story a few times, but when I was first making the tool, I was kicking myself the whole time, thinking 'why are you reinventing the wheel, there has got to be something out there that does this already.'   But I searched and searched and couldn't find anything, so I'm glad to hear you find the tool useful (perhaps showing that I was not crazy for writing it) and glad to hear it turned up when you searched for something (because I was in that spot myself not long ago ;)  
BTW Since I wrote this tool, two other intrepid programmers over on OpenGameArt have come up with their own takes on the idea.  You can find the results here:

https://repixel.app/

and here

https://williamthompsonj.github.io/Color-Palette-Swapper/

and a big thread discussing the whole topic here:

https://opengameart.org/forumtopic/tool-for-palette-swappingchanging

Finally, thanks for the feature suggestion!   And for the example picture, it really helps me understand what you're after.   I'll look into implementing this.   Internally, the program doesn't use Index mode at all, everything is full fat 24-bit RGB pixels, so there might be some challenges, but it does load indexed images and knows how to inspect the palette entries for an index image, so I'm sure I can figure out a way to get it working for you.

It is funny to say, because it's definitely not digging ditches, but I know what you mean about 32 clicks feeling like a lot of work!   :)

> Upon further inspection it seems the garbled images with the small pixels beneath them seem to be if the dimensions of the picture are too small.

The program uses OpenGL for display and a rule of thumb OpenGL likes even (mulitple of 2 or better yet power of 2) image sizes.  So it's possible the garbled pixels along the edge are just display issues caused by OpenGL quietly stretching the image to the next power of 2 or whatever behind the scenes.

I'll play around some with the image you provided and see if I can confirm that that's what's going on.

> Looks like the program crashes if the palette saved is only 16 colours, when it's 256 colours it's fine.

You hit the nail on the head.  Just dug through the code and in fact there was a crash bug pulling in images with a 4bit (16 color) index palette.

Pretty sure I've got that fixed now.  Going to keep looking on the other problems and hope to post a new build for you shortly.

> re: the out of order palettes:  

quick update after my first pass at figuring it out.   It looks like the program is reading those images in as 32bit RGBA images instead of indexed images.   Loading them in GIMP I can confirm that they should be indexed, so not sure what's going on.  

The program uses the standard C# image loader so something in that loader must be quietly reformatting the image.  Hopefully I can figure out a way to avoid that behavior...

Thanks so much for all the sample pictures!!   It really helps to have the exact data that's triggering the issue to test with.  Saves a lot of guessing.  :)

ok, I take that back about the garbled pixels.  It appears to be something more, looking into it....  

Thanks for looking into it. Since I've created the first post I've played around a lot with Grafx2, and it another program called mtpaint which have some very neat palette functions, but I'll say it again, your program once its bugs are ironed out would still be my go-to for this kind of thing.

haha, no worries if you find a better tool, just be sure to let me know about it so I can use it too!  :)

I've actually used both mtpaint and Grafx2 and like them both!   Grafx2 in particular is designed with indexed palettes in mind, so it does a much better job with them then GIMP imho.

btw, found the garbled pixels bug.  It did have something to do with OpenGL and 'odd' image sizes.  Although, as it turns out, not 'odd' as is in 'not divisible by 2' but 'odd' as in 'doesn't result in pixel data that perfectly aligns to 4 byte chunks'.   I've got a fix for it, hoping to look more into the 'palette loading out of order' bug and then roll out a bug fixed build soon.

thanks again for your patience and providing demonstration files!