This is close, but I want to set a user defined variable for how many months they want to go back.
Plus I want to highlight everything from 6 months back.
So if customer A hasn't purchased anything since December 2014 I want to be able to see that highlighted.
With the second part of your equation, it's limiting the scope to 1 year prior.
One solution I came up with was:
if( (Year (Max ( [Order Date] ) ) <> Year (Today () ) or ( inmonths ( varMonthLimit, Max ( [Order Date] ), Today (), -1) ) ) , Red() )
Which highlights everything NOT within this calendar year and X months back. Problem with this, is that in January, everything in December is highlighted red because it's not in the calendar year.