The ChangeColor plugin provides a dialog with two standard Java Swing color choosers, one for selecting a from-color, and another for selecting a to-color. The standard Swing choosers have been enhanced to include an extra tab for "Colors in use." The Colors-in-use tab displays a swatch for each color used in the model. Each color in-use swatch includes a hover tooltip that lists where the color is used.
Here is a screen shot where the top chooser shows the "Colors in use" tab with an active hover showing where color #E2D3DF is bing used. The bottom chooser is on the RGB tab, where E2E7BE is selected to replace E2D3DF.

Note that undo has not yet been implemented and I only wrote the plugin today. You should back up a copy of your model before using this plugin to make any changes.
I find the plugin quite helpful for temporarily changing a color to some other quite different and unique color, it then becomes quite apparent which walls and items do or don't have the right colors applied.
The plugin has been uploaded to
https://sourceforge.net/p/sweethome3d/plug-ins/14/
Installation is as per normal.
Future work might including implementing undo, but I'm not sure when I'll find the time. If anyone else would like to have a go at enhancing the plugin, the code base is relatively small.
Please remember to backup a copy of your model before playing with this plugin.