I tried and learned but not found a way to do what is needed.
Most importantly, even if the condition is met, the bars (or any chart) would change the color for entire series and not for a given bar (or interval). So even if we made the logic work, you would see the whole chart go one color at beginning of day, but back to normal afterwards. In other words, we don't have a way to change color properties of a chart just for one interval.
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I had another go at it and this is the best we can do, given the limitations; not sure if it serves you a purpose or not.
Basically idea is to have 2 chart series, one always showing 0 and other showing the actual value. Series switch the values at the beginning of (day/hour etc) and hence you get the boundaries. Drawback is that since one series is always showing 0, you have spaces between the bars. Rename the dpy.txt to dpy and have a look at how I achieved this. Any questions let me know.