Add option to color the lines not all tests ran

[JulianOrteil]

Is your feature request related to a problem? Please describe. In many of my projects, I like to use tox and tox-conda to test my source against multiple versions of Python. I've properly configured tox and coverage to test and generate an html report of how all tests went across versions. However, it is easy to miss the fact a Python version missed a line because the only indication it did so is the X ctx at the end of the line not being correct.

Describe the solution you'd like With the dropdown on the right, I think there should be an option to color the lines that were not executed by all environments. In the below example, I have 4 environments. Some lines are not executed by all environments. I think the visual cue for this could be improved.

Before

After

Describe alternatives you've considered N/A

Additional context I am currently running coverage 6.5.0.