Hi Hari
i hope the cell you want "Null" or other value to come is a result of
sum formula.

if it is so then you can use the following formula instead of
yourformula

=if( yourformula=0,"NULL",yourformula)

i m not very clear why you cant use it for the required purpose.if
above formula cant be used please elaborate the question. Well as per
my knowledge it is not possible to do with custom formatting or
conditional formatting.

Regards
NC