After an hour or so of messing around with it, I found a Solution!
So you have to have two rules.
First one is Cell is Empty, set the background color to white (not sure why this is needed, but it is crucial for it to work)
Second rule is Custom Formula is =len(B2)>150, set background color to red (this will set it to red if the their is over 150 characters.
This test spreadsheet really helped me out. (gotta make a copy to mess with it)
https://docs.google.com/spreadsheets/d/1ScDqtZvhsmf2iByAuxHBu0-ESpX_TE5pdtBvAxOJ99E/edit#gid=1620525099
Your welcome,
Chris
