You may have noticed this morning that when highlighting objects and lines in Revit that the lines are appearing Red. This will be the case if you are using Windows 7 - Last night there was a Windows update which has caused an error in the Revit UI which is making this bug appear. To resolve the issue, all you have to do is go into your 'Graphics options' and turn 'Hardware Acceleration' off. I presume there will be a future update from Windows that will sort this problem, but for now, this will temporarily fix the problem.