post-processing (data transformation) is one option
The dilemma you are describing sounds very similar to something discussed in this thread:
If so, then you have two options (as described on thread t=1907). The options are to: (1) create a list (usually a text list) for the “blank screen” event, or (2) copy-and-paste a simple spreadsheet formula down one new column that you create (after opening the saved data in Google spreadsheets or Excel or something similar).
If you have questions (or doubts) about applying those options to your experiment, then please attach your experiment here and we’ll continue to help.
It’s interesting that you mention a colour attribute, because that is exactly what is going on in our video demo on that prior thread. (this: http://www.screencast.com/t/mROe6OU3Vj ) In the video, cell G4 is compared to K4 (both on row 4). However, you could just as easily compare K5 to G4 (comparing across rows, from 5 to 4), and the formula would still be as quick to create and paste.