bypilot,
The JAA do not allow any mistakes 0 mistakes at either ishihara or the Baynes, Holmes Wright or Spectrolux lanterns. The Anomalascope is VERY hard and even a lot of colour normal pilots will not be able to pass to JAA standards.
The Tower Light Signal test is for FAA (USA) Transport Canada (Canadian) and CASA (Australia). It is NOT accepted by the JAA and nor are their medicals or licences.
In short if you want a JAA licence you need ZERO errors at either ishihara, Baynes, Holmes Wright, Spectrolux or a pass at JAA standards on the Anomalascope.
A word of advice. Try to get an "unofficial" test done before taking the real thing as if your country is like the UK, you will only have one chance. and the lantern tests are not easy!
All the best to you and your future career and never give up trying.