From what I have worked out, when the GTi models were launched the non turbo used all silver letters with the 1.8 Turbo models using the red 'i'.
When the non-turbo models increased in capacity they then became badged as 2.0 GTi (all in silver) and the Turbo models stayed as GTi, but with the red 'i' replaced with a silver one.
PD150 models are badged as GTi with the red 'i' and all other PD models are badged as TDi's
This means that all models are now identifiable by the badging on their bootlid alone, providing you know what year they are, as they are all running with unique badging on model by model basis.
However, for the nerds amongst us (myself included

) there are plenty of other visual clues as to which model is which!