Creating a conditional field based on two different dates from two different tables.

eilidh3 , you can create color measure like this

Color sales = if(AVERAGE(Sales[Sales Amount])<170,"green","red")
Color Year = if(FIRSTNONBLANK(Table[Year],2014) <=2016,"lightgreen",if(FIRSTNONBLANK(Table[Year],2014)>2018,"red","yellow"))

Color  = if(FIRSTNONBLANK(Table[Year],2014) <=2016 && AVERAGE(Sales[Sales Amount])<170 
,"lightgreen",if(FIRSTNONBLANK(Table[Year],2014)>2018,"red","yellow"))
Color sales = if([Sales Today] -[sales yesterday]>0,"green","red")

, based on your conditions and use it after choosing field option in color grouping

For date across two table , how to handle refer :https://community.powerbi.com/t5/Community-Blog/Decoding-Direct-Query-in-Power-BI-Part-2-Date-Difference-Across/ba-p/934397#M451

Formatting

https://radacad.com/dax-and-conditional-formatting-better-together-find-the-biggest-and-smallest-numbers-in-the-column
https://docs.microsoft.com/en-us/power-bi/desktop-conditional-table-formatting#color-by-color-values