Why would Color-enabled themes work for all sites but one?

The problem: Themes that are "recolorable" with the Color module are working fine on my sites B, C, D, etc., but not site A. On that site, *choosing* a new color scheme works fine, but those new colors just won't show. Here's why it's strange: 

* All sites are under the same Drupal 5.x installation, and are all set up pretty similarly; there's nothing special about site A.
* All sites have Theme API and Color module enabled. (Theme API may not be necessary, but disabling it doesn't solve the problem.)
* All sites have the file system set to Public, as required for the Color module to write to certain directories.
* On all sites, even site A, "recolorable" themes show the color wheel on the theme config page, and allow color modifications.
* On all sites, Drupal is properly writing to sites//files/color/. Even on site A, Drupal creates new colored graphics within that directory, like it should.
* On all sites, permissions are identical for all directories in the above path. New graphics created for "recolorization" also have the same permissions for all sites.

So, it would seem that the relevant factors are set up equally for all sites, and all infrastructure seems to work; even site A lets me create new color schemes, which display nicely as a preview on the theme config page, and the appropriate new graphics appear in the file structure as they should.

Yet site A seems to refuse to *use* those new graphics, and will only display the theme's default colors. It's not a single theme issue; it's true for both Garland and Pixture. I don't think it's a cache issue; the problem sticks through multiple browsers, log-ins, a cache clearing, even a site update.php. It's not a user permissions issue; all sites are tested as ID 1, and there's no special admin theme for any site.

So: What's a possible reason why only one site would allow *setting* new colors in themes like Garland, but refuse to *display* those colors?