Making Sum by ColorFunction automatic

Question

Thanks in advance for any help on this that you can offer.

With the help of this site I have created the following function;

Function ColorFunction(rColor As Range, rRange As Range, Optional SUM As Boolean)
Dim rCell As Range
Dim lCol As Long
Dim vResult
lCol = rColor.Interior.ColorIndex
If SUM = True Then
For Each rCell In rRange
If rCell.Interior.ColorIndex = lCol Then
vResult = WorksheetFunction.SUM(rCell, vResult)
End If
Next rCell
Else
For Each rCell In rRange
If rCell.Interior.ColorIndex = lCol Then
vResult = 1 + vResult
End If
Next rCell
End If
ColorFunction = vResult
End Function

In each target cell that I want a sum of all the of the cells in that row that contain the color of the specified cell;
=colorfunction(AR4,H5:AP5,TRUE)

How can I get the target sell to update when any other cell in the row (H5:AP5) changes color?