Go to Properties of Text Box
Select Color under Background section
Use Calculated under base color and write below expresssion
IF(WeekDay(Today()) = 'Mon', Green(),
IF(WeekDay(Today()) = 'Tue', Red(),
IF(WeekDay(Today()) = 'Wed', Blue(),
IF(WeekDay(Today()) = 'Thu', QlikTechGray(),
IF(WeekDay(Today()) = 'Fri', QlikTechBlue(),
IF(WeekDay(Today()) = 'Sat', RGB(100,100,100),
What I can suggest (i did it this way for one application with products where each product (about 12) should always have a defined colour) is, to nhance the datamodel (in my case I added a column "Productcol". I used if statements for each product. In my Text colour properties I could use this field "Productcol").
-> You need only one expression other than several if statements for each expression where you need these colours. In your case can you enhance a table with these colours (maybe the calendar or a new table attached to calendar)?
But if you have only one expression I would suggest like Manish description. If you have more than one, you migth consider my solution. Its always a question of maintain and work to do.