In Excel 2007 I am trying to change the Fill color of a cell from a macro. I
am fairly adept with excel macros. So here is the problem with a simple
example.

Sub FormatResult()
 Range("A1").Interior.Color = 255  '*** This Works
End Sub

Add a formula to a cell e.g. =FormatResultByRange(A1)
Function FormatResultByRange(r as Range)
 r.Interior.Color = 255 '*** Error 1004
End function

alternatively...
Function FormatResultByRange(r as Range)
 ' r is ignored and not used
 Range("A1").Interior.Color = 255 '*** Error 1004
End function

It doesn't seem to work when executed via a formula.

Code In Context:
I know the follwoing example can be achieved using coditional formatting,
but the real code is actually very complex involving 3d math etc. But what is
below illustrates the problem in context.

n.b. cell formula:  = FormatAge()
Function FormatAge()
 Dim r as Range
 Dim age as Integer
 Dim fmtcolor as Variant
 Set r = Application.Caller
 age = r.Value
 if(age<16) then
    fmtcolor = &H0000FF
 else
    if(age<18) then
       fmtcolor=&HFF8000
    else
         fmtcolor=&H00FF00
    end if
 end if
 r.Interior.Color = fmtColor '*** Error 1004
End Function