I created custom inspection error level (JAVADOC WARNING) with gray underwave and gray tickmark. I assigned it to General -> i) Declaration has javadoc problems and ii) Declaration has problems in javadoc references.
The underwave and the tick mark are not displayed correctly in the editor screen. The file inspection color box (top right corner) is no longer red, but the individual errors are still highlighted using the ERROR styling instead of mine. Specifically, the error was Cannot Resolve Symbol xyz, and the lightbulb popup would correctly identify it as 'Inspection Declaration has problems in javadoc references.'
If I assigned INFO level to those two entries in the profile, then the highlighting and tick marks follow the INFO setting. So, it seems that custom highlighting doesn't work properly.