Hi nubehaviour,
Do you mean 1 month = 4 weeks = 28 days?
And 6 months = 168 days?

You can use Conditional Formatting with formulas:
F3: =MOD(TODAY()-E3,7)=0
G3: =MOD(TODAY()-E3,14)=0
H3: =MOD(TODAY()-E3,28)=0
I3: =MOD(TODAY()-E3,168)=0

Choose color you want for each.

If a month isn't 28 days and 6 month isn't 168 we use other formula.