Trying to get SumByColor working

You should be using Double not Long as your numbers are not whole numbers. You also need to make the function volatile, and it's better not to pass the cell the function is in as an argument:

Function SumByColor(CellColor As Range, rRange As Range) As Double
    Dim cSum                  As Double
    Dim ColIndex              As Integer
    
    Application.Volatile True
    ColIndex = CellColor.Interior.ColorIndex
    For Each cl In rRange
        If cl.Interior.ColorIndex = ColIndex Then
            cSum = WorksheetFunction.Sum(cl, cSum)
        End If
    Next cl
    SumByColor = cSum
End Function

Of course it's also far better not to use colours as data.

Your reference cells in row 3 to pick up the colorindex are conditionally formatted, which is not picked up by colorindex. And CF will change the color without triggering a re-calc, so that self-referential technique is just a bad idea.

If you are using CF to find the cells to sum, then you should use a formula that replicates the CF rule. If you are manually coloring the cells, then instead of referencing a cell that uses CF, use a cell that is also manually formatted for the color of interest.