How to color code individual data points in line chart
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Unfortunately, I am not aware of an easy way to do what you are asking for in Numbers. Below is how it could be done in case you wish to do it no matter how:

In the example above, the data for both high and low blood pressures are shown in three separate columns, assuming there are three time zones in a day to be highlighted. I used the formula shown in the picture to separate the readings by time zone (you can do all of the separation manually if you like but doing it all manually might turn out to be extra tedious and it might be cumbersome to apply new time zone limits if you change your mind). Of course, the formula is slightly changed for each column so that it refers to the correct cells. In addition, the first non-zero reading in each second and third columns is overwritten manually with the number to its left to get continuity in chart lines (the second 73, 72, 143 and 142).
I used the Format -&gt; Style sidebar menu option to set the high and low lines with the same color after selecting the respective part of each line as shown in the picture below:

I used a relatively old version of Numbers (6.1) to produce the table and the chart above but I reckon you would have no trouble producing them as well in case you have a newer version.
Hope these help!