Please try "Editor | Color Scheme | General > Hyperlinks > Inactive", that should help.
Also, a piece of useful advice on how to detect responsible part in color scheme -- put the caret on the place that you are interested in, invoke "Find Action" (Ctrl+Shift+A on Windows/Linux) and type "Jump to Color and Fonts".
No, unfortunately, preview pane in Color Scheme configuration is read-only, you could not edit it.
Also, there is no known way to use different colors for attribute value based on its contents, sorry.
Please feel free to submit feature requests on YouTrack:
https://youtrack.jetbrains.com/newIssue
Any idea how to change the colour of the underline here?
Please try "Editor | Color Scheme | General > Hyperlinks > Inactive", that should help.
Also, a piece of useful advice on how to detect responsible part in color scheme -- put the caret on the place that you are interested in, invoke "Find Action" (Ctrl+Shift+A on Windows/Linux) and type "Jump to Color and Fonts".
Thank you so much, that is SUPER useful!