Sounds like a simple question I know, but here is the basis for the question. Like most packages, my software will color code volume. I have red for down days and black for up days. I notice, however, that the color coding is based on the relationship of the close to the open vs the relationship of today's close to yesterday's close. For example, you can see with KLAC that there are days where the stock gapped higher, closed lower, yet the close is still higher than the previous day's close. I haven't checked to see how this relates to OBV, because I don't use it. But I wonder about the accuracy. So what constitutes a down day?