Hi Herbert,
Yes, the ROI is already in our list of upcoming features for this tool and, in fact, probably will be the next update. The ROI is also needed because it can be problematic to measure color inside some extended objects. For instance, we have had problems measuring color in a picture of M31; not because PCC fails, but because M31 is a very complex object and it's difficult to do the photometry in the same way as it has been done in the star catalog. For that picture, a ROI outside the galaxy works very well.
Best regards,
Vicent.