Automatically Run Udf When Color Is Changed

I have a UDF that goes like this:

Function SumIfPurple(inputRange As Range, _
answerRange1 As Range, _
answerRange2 As Range) As Variant
Dim SumAnswer As Variant

'If inputRange is turned purple then it is equal to anserRange1, if it is left with no fill it is equal to answerrange2.

If inputRange.Interior.ColorIndex = 39 Then
SumAnswer = answerRange1.Value
Else
SumAnswer = answerRange2.Value
End If
SumIfPurple = SumAnswer
End Function

I would like to have a sub that will have the formula automatically calculate when the inputRange's color is changed.