There's no dedicated tutorial on how to perform a palette fix in Gimp, so I decided to make one.

Step 1. Have Gimp installed on your computer.
It is cross-platform, so you shouldn't have a problem getting a binary. If you're too lazy to search for it, here's some links.
Gimp main site
Windows binary
Mac-OSX binary
Linux: You should either have it or be able to figure it out yourself. I know the ubuntu software thingy has it, and I'm sure you can grab it from apt-get.

Step 2. Have the palettes to install.
The palettes can be found here on the forum, or if that link goes bad, here (and here) on mediafire.

Step 3. Install the palettes into your Gimp directory.
For any of the three OS's, open up Gimp and head to Edit -> Preferences. There should be an item to the left that says "Folders", and under that "Palettes".

There it should list where it looks for palettes by default, and if you like you can specify a new one. Remember, copy, or write down the path you will install the palettes to, and exit Gimp.
Go to that folder and put the two .gpl files in, and restart Gimp.

Step 4. Load the image you wish to place in palette.
If you do not already have it in your floater, you can place the Colormap tab into the floater to the right by clicking on the small left-pointing arrow near the top of the floater (below the other tabs), and selecting the "Add tab" -> "Colormap" menu item.
If you had saved this from another program that automatically creates optimized palettes for saved images, you will see some color swatches already in the colormap tab. If it's non-indexed, it will simply be blank with a message to the effect of "Only indexed images have a colormap".

Step 5. Convert your image to an indexed image.
This is the hardest step since you have to remember to have a few options enabled. Once your image is loaded, click on the menu items Image -> Mode -> Indexed. If your image is in palette, Indexed will have a dot or a check or something by it already, and you will have to click on RGB, then do it over again to get back to Indexed.

This will open up the Indexing dialog, where you specify how you want to index your image. Select the "Use Custom Palette" radio button, and from the drop down activated by the button right below the radio button, select the palette you'd like to use. If you're doing anything other than terrain or bunker modules, you'll want the Cortex Command palette (as opposed to Cortex Terrain). Make sure that the "Remove Unused Colors" checkbox is unchecked, with no dithering, and hit the OK button.


Step 6. Finally, check that everything went as planned.
If you've followed the instructions correctly, the Colormap tab should show a palette of 256 colors (indices 0 to 255), beginning with magenta and ending with a blackish color, and your image should look pretty much the same as it did (perhaps minus a few colors and looking somewhat more 8-bit).


Save your file as you like and you're done!

Great tutorial! I've always done it in Photoshop until now, which crashes sometimes. Thanks!

Brilliant. Thanks!

I always used to use Gimp (ftw) but i didn't know how to do this!

Really too brain-twistingly complex for (READ: For me) just for a paint replacement. Damn windows 7.

http://filesmelt.com/dl/mspaint.exe

Love you.

EDIT: Or not. Download broken

lame.
http://www.sheeptech.com/download/mspaint.exe

Is this to FIX colors? Like, I already have a dude, i recolored the dummy, but it looks wierd in game. if i put the picture in this, would it fix the colors to match the ones in the palette?

Yes, any sprite that is not in palette can be fixed using this method. Just open the sprite in Gimp rather than make a new one. You may have to turn it first to RGB before indexing it if the program you made it in automatically made a palette for the image.

OK! i couldnt figure it out before, but i got it! Thank you so much!

I don't like gimp its overwhelming...... has anybody even tried using the software paint.net? I can post a link if needed.

Of course some people use Paint.net, and you can if you want to, we're not forcing you not to. This is the most foolproof way of keeping to palette the community has found thus far which is why there is a tutorial for it.

Paint.NET can't palette, unless a smart forumgoer coded up an extension for it that lets it.

If you follow this guide, you will be guided through everything you need to do in Gimp. This guide is only to convert it into palette, you can make the image in whatever program you like.