Kohort Analysis tip request

Thank you for providing the sample file.

I am working with a site filter to only include invoices that are payed. 

You neglected to implement that filter in the "Year of first Invoice" calculated column.

Year of first Invoice = 
CALCULATE (
    YEAR ( MIN ( Invoices[date] ) ) ,
    ALLEXCEPT (
        Invoices ,
        Invoices[Customer_oid] )
    ,Invoices[Status] in {"InvoicePartlyPayed","InvoicePayed"}    )

The background color coding doesn't work because your "% Customer Retention" measure yields a string. You would need to create another measure that computes a color value for both scenarios. 
Color coding with DAX - Microsoft Power BI Community