You can't change the cross cursor (set on the Display Options form) colour dynamically. If you do:
virtuoso -xrm "Opus.dragColor:red"
you can change the colour at startup, but it can't be changed during the session. To do that would require an enhancement - so if you want that, please contact Cadence Customer support. There is an enhancement CCR to set the colour already, CCR 1094757.