One way:
Public Function SumNoColor(ByRef rng As Range) As Variant
Dim rCell As Range
Application.Volatile True
For Each rCell In rng
With rCell
If .Interior.ColorIndex = xlColorIndexNone Then _
If IsNumeric(.Value) Then _
SumNoColor = SumNoColor + .Value
End With
Next rCell
End Function
Note that this will not automatically update when a background color is
changed. The Application.Volatile statement will recalculate the
function whenever the worksheet is recalculated (including when you type
F9).
If you're unfamiliar with UDFs, see David McRitchie's "Getting Started
with Macros and User Defined Functions":
http://www.mvps.org/dmcritchie/excel/getstarted.htm
