Conditional formatting color change if value goes down

Fragan , You can use time intelligence and date table for This Month Vs last month

MTD Sales = CALCULATE(SUM(Sales[Sales Amount]),DATESMTD('Date'[Date]))
last MTD Sales = CALCULATE(SUM(Sales[Sales Amount]),DATESMTD(dateadd('Date'[Date],-1,MONTH)))
last MTD (complete) Sales = CALCULATE(SUM(Sales[Sales Amount]),DATESMTD(ENDOFMONTH(dateadd('Date'[Date],-1,MONTH))))

Use color grouping on Diff

https://docs.microsoft.com/en-us/power-bi/desktop-conditional-table-formatting#color-by-color-values

Or create a color measure and use it with "Field" option in color grouping

Color sales = if([Sales MTD] -[sales LMTD]>0,"green","red")

https://radacad.com/dax-and-conditional-formatting-better-together-find-the-biggest-and-smallest-numbers-in-the-column