Is there a way to do a simple conditional format on a matrix?

HI All

I have created a matrix table which shows year first, then month then a list of customers who have paid or not paid during that month.  I would like this visual to stand out a bit more by flagging those that have not paid to have the cell in some color, so it will be more visible to users who has not paid and who has.  Is there a simple conditional formatting rule that can be created and applied to the table to allow for this?

Emma