Public Function SumByColor(rng As Range, clr As String) As Double
Application.Volatile

Dim s As Double
s = 0
Dim r As Range
Dim c As Long
If clr = "red" Then
    c = RGB(256, 0, 0)
End If
If clr = "black" Then
    c = RGB(0, 0, 0)
End If

For Each r In rng
    If r.Font.Color = c Then
        s = s + r.Value
    End If
Next r

SumByColor = s
End Function