Girls und Panzer: Dream Tank Match - Farming Decals and Tank Colors
12https://psnprofiles.com/guide/7155-girls-und-panzer-dream-tank-match-trophy-guide
If I count it correctly, there are 716 decals and 99 tank colors in the game. 29 out of 99 tank colors are from other tanks' color (just unlock these tanks to unlock their tank colors for use on your fav tank), and some decals are available at the beginning as default (assume there are 16 of them). It seems that patchwork BOCOs give tank colors while the rest give decals.
You can grind it in Story Battle #1, select Miho Nishizumi since she has better view than Darjeeling's, and your allies rarely destroy the targeted tank before you hunt all Bocos in this stage. Don't worry about it may take more time, because you have to play the game (match time) more than 20 hours for its trophy anyway. This trophy and level 80 trophies are likely your last trophies. 5 items (decals or colors) per match, you may expect to play ~151 matches for this mission if you haven't hunt any Boco before.
It seems that every map has 3 different patterns of BOCO location, only one or two BOCOs will locate at different area while the rest of BOCO have fixed location. You can see the different patterns on mini map during the first few seconds in the video.
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Yuki Hasegawa presently has 9,972 views for Girls und Panzer: Dream Tank Match across 14 videos, and about an hours worth of Girls und Panzer: Dream Tank Match videos were uploaded to his channel. This is 7.42% of the total watchable video on Yuki Hasegawa's YouTube channel.