Conditional formating on formala cells

Hello,

i m trying to apply the conditional formating on the cell which is containg the formula,

a b c d e f g h i
23 24 25 26 27 28 29 30 31
8 8 8 F 8 8 8 8 8

all the above cells are automated using the formula, this is a time sheet, if the month eneterd Feb, then the cell information from g to i will be vanished. My requirement is when these dates get vanished it has be get filled with some color using the conditional formating. when i am trying to use conditional formating its not effecting on the cell.

formual contained in the cell g is =IF(ISNUMBER(AD8)=FALSE,"",IF($AI$7="FEB","",AD8+1))

can you please help me to acheive this.

SALEEM