You need to add another condition to select the last column:
if([solar_field_installation...]='Dec-215' AND KW<0.6*(Before(KW))and KW<>0,Yellow(150),if( KW>=1.4*Before(KW),green(150),if(KW=0,red(100))))
Replace solar_field_installation... with the real field name of the chart dimension.
If you need something more dynamic than a hard-coded value like Dec-2015 then perhaps something like Max(Total [solar_field_installation...]) will work.