Lines in Erb templates that start with % are considered to be ruby until the next new line (with the appropriate Erb options). Netbeans does that correctly, 
except when the lines start with spaces before the %, then Netbeans still colors them as ruby code even though the are not. For example:

% begin # this is ruby
  % this is not ruby
% end 

will colorize "this is not ruby" as ruby code.
On a related note: the Source -> Format command inserts spaces before the % in Erb templates making them malfunction