I saw a new facelift '7.5' 1.6 TDI with 'D' and 'I' in red a couple of weeks ago. I've never
seen a 'TDI' in blue yet. In the past, some 'FSI' Golfs had the 'I' in blue though, and the
current eGolf and GTE hybrid are using blue letters. I'm under the impression, Volkswagen
is using blue for electric and hybrid verhicles currently.
There's also been green for the Lupo 1.2 TDI '3L', perhaps 'D' and 'I' in green. I don't think
they were using green color anywhere else.
Generally, in the last 15 years VW didn't use colored 'TDI' badges that often, since the
different engines could be easily distiguished by using a simple 'TDI' for the 1.6 litre and
'2.0 TDI' for the two-litre Diesels.
In the very past (> 15 years) where all their 4-pot Diesel have been 1.9 litre displacement,
Volkswagen did it this way, in particular on the Golf Mk.IV:
TDI - 90 HP
TDI - 110 HP (and 100 HP Pumpedüse from mid 2002 on)
TDI - 115 HP Pumpedüse (from 2000 on)
TDI - both 100 and 130 HP Pumpedüse (from 2001 on)
TDI - 150 HP Pumpedüse
.