I have a couple of panels which are refreshed separately from the page and this can take some time. In order to give the users a cue something is happening I display a twirly animated gif till something more interesting can be shown.
However it has the wrong background color. Is there some tool to efficiently change the color in the pallettes so I can turn it from white to pale blue, and keep the interpolated colors to reduce the jaggies consistent with the background color?
Maybe you could try to create the loader gif with this classic service: http://ajaxload.info/
It has options for transparency.
Or I've used http://www.preloaders.net/ - also has options for transparency but a few more options...
You can define background-transparent-color of gifs in final "save for web" export in PS. In gimp it's quite harder, but also possible, try this http://geekswithblogs.net/timh/archive/2006/03/20/72797.aspx