VW Golf GT TDI GTI Badges - What do the Red I Badging colours mean?

There is a lot of confusion over the badges that appear on VW Golf mk4 TDI cars as there is so much variation. What do the colouors on the badges on the VW Golf Mk4 TDI signify?

My car is a VW Golf Mk4 with 150 bhp GT TDI PD engine and the badge on the back is purely GTI as below:

 mk4 gt tdi badging

Yet most expectations are that GTI only appears on a PETROL VW Golf not a diesel. On a petrol GTI the red I signifies that it is the turbo charged version.

For the Golf Mk IV TDI models the 150PD can also appear with the badge showing GT TDI or TDI all in red - there doesn't seem to be any pattern!

TDI - all in silver 90/100bhp

TDI - 110 bhp

TDI - red DI PD 115 or PD 130bhp

TDI - red TDI PD 150bhp

GT TDI - PD 150 bhp

GTI - red I, either 150 bhp petrol or 150bhp diesel TDI PD