Cell colored based on another cell

[BADebbie]

I'm sure this has been asked a hundred times, but I can't seem to figure it out.

I would like to fill a cell with a color based on another cell if it says "Mon". The cell that has "Mon" in it, has custom formatting to show the day of the week based on the date above it. I don't know if conditional formatting will work with the IF function because it's not truly text. I have attached my spreadsheet. Line 25 and 28 formatted based on line 3 days.

Example; if b3="Mon", b25 is blue. Actually if it is Mon, Tues, Wed, or Thurs it should make the b3 cell blue. Otherwise it should be gray.