Matrix heatmap fixed background color

I have a report that has risks with the impact and likelihood rating. In table format that looks something like:

With the matrix visual I was able to get the count of the number of risks per area plotted the way as shown below. Now I only need to get the background formatting done. This should always be the same background color, independent of the number in that particulair cell. So the top right cell should always be dark red, bottom left dark green, etc. Is there a way to get the background sorted this way?