Function Recalculation With Change In Cell

I have just written a function that sums all the values in the cells in a range that are not green. This works however if one of the non green cells is changed to green the function does not work. i have to re input it into the cell that i put it into.

Function SumNotGreen(SelectedCells As Range)
' Adds the values of the cells where the font colour is not green(35).
Dim Cell As Object
Dim x As Double
x = 0
For Each Cell In SelectedCells
If Cell.Interior.ColorIndex <> 35 Then
x = x + Cell.Value
End If
Next Cell
SumNotGreen = x
End Function

How can i make the function recalculate i.e. go back into the loop every time a change is made.