Matrix subtotals average and sum

Hello All

I've got a matrix visualisation that I'm trying to get the sub-totals to be either sum or average depending on the level:

Product Group (row)

   Product Name (row)

      Version (row)

         Colour (row)

            Quantity (value)

I'd like the subtotal of the colours to be the Sum of the quantity  and for the other rows the subtotal should be the average of the sum in the colour subtotal

I've tried various custom measures and can get some of the right subtotals but not all.  Any help appreciated.