Add a button on Replace Color window to easily and quickly swap Foreground and Background colors with each other.
(This button could also be placed somewhere near the main Foreground and Background color indicators.)
This feature would help to speed up user workflow. When replacing colors, if your Foreground and Background colors are backwards from what you are trying to replace, the user must close the Replace Color window and manually change both colors by selecting them on the color palette and then re-select Replace Color.
(This feature exists in Photoshop I believe.)