simple formula,but I' can'f find it anymore

[Belleke]

You have an number in Cell D18, if cell in cell D21 is empty cell E21 should be empty, if cell D21 has a value cell E21 should give de difference between D18 and D21.
If positive in black if negative in red.
See example.

 

[ETAF]

try
=IF(D18="","",IF(D21<>"",D18-D21,""))

I'm assuming its D18 - D21 , or it could be D21-D18 - as you just said the difference

What do you want coloured ?? you are showing black and red border in D18 and D21

not sure if the border can be coloured

But using conditional formatting - you could change the cell colour or font colour
OR
you could use a format to show that
BUT just not sure exactly what you wanted for that

 

[vletm]

Something like this?

 

[Belleke]

Thank you @ETAF and @vletm .
problem solved.
Both you're suggestions are appreciated.

 

[ETAF]

you are welcome