Recalculate formula when format changes

Hello,

I have a VB module that sums cells with a specific color background. It works fine but I have to click on the cell with the formula to get the module to recalculate the sum whenever the color of a cell in the defined range changes. How can I get this to run automatically? Here's the VB module:

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

Thanks!