Something like this should work:
Condition
translate(field, ":-T", "") <= translate(xdDate:AddDays(xdDate:Today(), 7), ":-T", "")
Shade green
Condition
translate(field, ":-T", "") > translate(xdDate:AddDays(xdDate:Today(), 7), ":-T", "") AND translate(field, ":-T", "") <= translate(xdDate:AddDays(xdDate:Today(),30), ":-T", "")
Shade amber
Condition
translate(field, ":-T", "") > translate(xdDate:AddDays(xdDate:Today(), 30), ":-T", "")
Shade red