VBA AutoShape Color Change Script for Formula Cell

VBA will pick up the value by default in a term such as:

If Range("U117") = 1 then

The intersect function determines whether the ranges overlap, and the arguements for the function must be ranges. Therefore you cannot have the resulting formula of A2 - A1 instead of range("U117").

If you want to determine whether the value of "target" is equal to the value of A2-A1 you need:

Private Sub Worksheet_Change(ByVal Target As Range)

If Target = Range("A2") - Range("A1") Then
If IsNumeric(Target.Value) Then
If Target.Value > 0.4 Then
ActiveSheet.Shapes("FreeForm 9").Fill.ForeColor.RGB = vbRed
ElseIf Target.Value <= 0.4 And Target.Value > 0.05 Then
ActiveSheet.Shapes("FreeForm 9").Fill.ForeColor.RGB = vbYellow
ElseIf Target.Value <= 0.05 Then
ActiveSheet.Shapes("FreeForm 9").Fill.ForeColor.RGB = vbGreen
End If
End If
End If
End Sub

This worked for me
NB:- If you assume the results of formula cell is "A3" and the "Precedents" cells are "A1 & A2",
Then "A1 :A2" will be the Actual target cells, Not "A3"
The result of formula cell (in you case "U117" ,in Example case "A3") is not the Target cell
(NB:- Target cell= Cell selected)
But by setting the Cell ("U117 or "A3") as a range object "Tar" it can be used in the code as below.
Example code below based on your Code, but with alterations:-

Private Sub Worksheet_Change(ByVal Target As Range)
Dim Tar As Range
Set Tar = Range("A3") 'Formula result cell
If Not Intersect(Target, Range("A1:A2")) Is Nothing Then
    If IsNumeric(Tar.Value) Then
        If Tar.Value > 0.4 Then
            ActiveSheet.Shapes("FreeForm 1").Fill.ForeColor.RGB = vbRed
        ElseIf Tar.Value <= 0.4 And Tar.Value > 0.05 Then
            ActiveSheet.Shapes("FreeForm 1").Fill.ForeColor.RGB = vbYellow
        ElseIf Tar.Value <= 0.05 Then
            ActiveSheet.Shapes("FreeForm 1").Fill.ForeColor.RGB = vbGreen
        End If
    End If
End If
End Sub

Excellent.
Thanks for the advice guys. All working now, and I've learnt something new.
Cheers

If in the future I want to duplicate this script to act on additional different cells (and their corresponding freeforms) within the same worksheet, is there a VBA procedure I need to separate them?
(Sorry, very new to this)