>IIF (THISFORM.text18.VALUE <0,THISFORM.text18.FORECOLOR=RGB(255,0,0) , THISFORM.text18.FORECOLOR=RGB(0,0,0))

What your code is doing is evaluating if the current forecolor equals read, and then returning the logical value to nothing. IIF() evaluates and returns the 2nd and 3rd parameters, it doesn't execute them.

So, what you want is this:

THISFORM.text18.FORECOLOR = IIF (THISFORM.text18.VALUE <0, RGB(255,0,0) ,RGB(0,0,0))

BTW, the space after the IIF doesn't make a difference.