The Attribute Masking tool is dynamic display tool which allows resymbolizing of selected elements based on certain properties.
The tool settings provides a wide number of display options. For example, quickly see which elements are inferred features, or which are in an attached reference.
The colour used to highlight the mask is set in your Workspace > Preferences > Symbology > Mask option.
For more information on the use of this tool, refer to:
Update March, 2019: At this time, attribute Masking is not available in OpenCities Map CONNECT Edition Update 3