UPDATE: CyberDoc and Mike Mills kindly recommended the freeware PhotoFiltre (installer-free version available), which worked every bit as well as SnagIt. It's as simple as clicking Adjust > Replace color and using the droppers to select. Initially, most menu items were grayed out for the GIF I had opened, but Mike informed me of the solution: Image > Mode > RGB color. Thanks to you both!
/windows | Apr 12, 2009