Map Shading wot a twist

I am trying to recreate a Tableau chart in Power BI based on the first 30 seconds of this video: https://www.youtube.com/watch?v=z67mhbUlkmw

I currently have a date filter in Power BI to change the bounds on a line graph showing the cumulative COVID cases and the figures on the map. Dark red on my map corresponds to the highest number of cumulative cases for that day. So for example, red corresponds to 34 in March 2020 but a much higher number in May 2020.

I am trying to have the darkest red shading only be applied in May 2020. Does anyone know how I could do this or perhaps have an example workbook to follow?