Painting Monday....

To paint the background of your chart for a specific day of the week, first create a signal that identifies that day of the week.  Using the Position indicator....

POS = 1

Setup POS with "Day of Week".  0 represents Sunday, 1 is Monday, .... 6 is Saturday.  So POS = 1 will identify all the bars on Monday.

Save the signal with a name (I called this one simply "Monday"), then click on "Send To.... > Chart: Paint Bars/Background" as seen below.

In the PaintBar settings, choose "Paint Background (Top to Bottom)" so it paints the full pane background for each bar.  Check the "If True" checkbox and specify the color.  If your chart is white, you'll want a very light color.  If your chart is black, a very dark shade of a color.

If you want to paint another day, say Thursday, you would repeat this process with POS = 4.  If you want this one PaintBar to paint multiple days, say both Monday and Thursday, just edit the signal to....

POS = 1 OR POS = 4

Here is the chart definition for the chart above: https://www.linnsoft.com/charts/paintmonday-es