You know what? I'm keeping this permanently on hand, because this happens too often.
HALP! My sprite looks all weird ingame.. and when I select it, the game crashes! I checked the sprite in a graphics program and it looks fine! WTF?!
Like this?
That is because your sprite is not 8-bit and doesn't match the CC palette. I won't tell you what 8-bit is. If you don't know, Wikipedia it.
Simply decreasing the color depth to 8-bit won't help, you'll only have the colors that the sprite uses. It won't match the CC palette. 1 of 2 requirements. Sure, the mod won't crash now; but your sprite will look like an acid trip.
To get it to match the CC palette, memorize the dimensions of the screwed up sprite. 34 by 12 pixels, for example.
Next, make a copy of palette.bmp (in Base.rte) and open it up in a graphics editing program. Resize the copy of palette to the exact dimensions of your sprite.
Next, copy the screwed up sprite into the palette copy, having it fit perfectly over all of the colors.
Save, and there! You now have a sprite that's 8-bit and has all the CC colors. You'll have to do this for every custom sprite that acts the same way.