Backdrop versions prior to 1.11.0 included image handling capabilities in Color module that allowed the creation of gradient images for themes that include Color module support.
However, there are no known themes that have used this capability in Backdrop. In Drupal, only the Garland theme was known to have used this ability. As this capability was not used and included no test coverage, it has been removed from Color module. Themes that need to generate gradients should use CSS gradients instead of image-based ones.