Color Accent

Color Accent is a small-sized extension specially designed for Paint.net, the advanced image editing application. It enables you to keep a specific color of the picture while converting the remaining area to greyscale.

Simple integration with Paint.net

Installing this plugin is a simple task, since there is no actual setup pack involved. Instead, you can download and copy the DLL files to the "Effects" folder of the photo editor. It doesn't matter whether you have an installed or portable edition available, since the add-on works regardless.
If Paint.net was already launched prior to this step, it has to be restarted to finalize setup and gain access to the new plugin. It can be found in the "Adjustments" menu and its entry is accompanied by a tiny icon that looks like the piece of a puzzle. This is an indicator for all external plugins of Paint.net.

Convert to grayscale while retaining a color

After opening the image, you can bring up the configuration panel of Color Accent to be able to pick a color from the color wheel or by entering its RGB channels, restore it to default, tweak the level of tolerance, as well as preview modifications to decide if you want to keep them or not.
There is no built-in feature for choosing a color straight from the image, so you can either match one from the color wheel or use another color picking tool prior to this extension.

Evaluation and conclusion

Although Color Accent hasn't been updated for a long time, we haven't experienced any compatibility issues with the latest version of Paint.net in our tests. It didn't cause the program to freeze, crash or show error notifications. The modified photo was quickly rendered in the preview mode and after committing settings.