All right.
You need to go through a few steps to achieve this. The following is based on the assumption that columns are dates only, and that no gaps exist and that the first column is a Monday (can be changed).
1. TARGIT cannot color empty cells, so first you need to copy all the original values into a calculation. Add a custom calculation, as a new measure:
a. sum(0, 0, m1)
2. Go to Visibility (on the Properties Smartpad) and hide the original measure.
3. Optional: Format numbers (Properties Smartpad) of the new calculation to #.##. This will show zeros as blank, if you like.
4. Add a color agent to the new calculation:
a. count(0, 0, m1) = 0 and mod(allcount(d1:0, d1, m1);7) <> 0 and mod(allcount(d1:0, d1, m1);7) <> 6
5. This will color the empty cells, except for two columns every 7 columns (Saturdays and Sundays). If your table is starting on e.g. a Wednesday, you change the syntax to:
a. count(0, 0, m1) = 0 and mod(allcount(d1:0, d1, m1);7) <> 5 and mod(allcount(d1:0, d1, m1);7) <> 4
BR / Ole