I got a reply from Nintendo germany regarding my question about the coloured traingles. They told me that these triangles represent the languages included in the manual (which we already knew) and that this issue shouldn't be of any interest to me (no joke, that's what they said, they basically told me it's none of my f'n business). Maybe Nintendo UK is more helpful when you send them an email. I doubt it though.
So, again, we have to find them out ourselves. Going through my collection of Wii, GC and DS games I can confirm that Germany is dark blue and UK is dark green.
My Capcom Vs SNK 2 EO for Gamecube has a light green triangle and German and Spanish text on the back and in the manual as well as the German and Spanish age rating.
WWE Day of Reckoning has a pink triangle and has German and French text on the back and the manual is also in French and German.
Curiously enough my Castlevania Dawn Of Sorrow also has a pink triangle but has English, German, Spanish, Italian and dutch included in the manual. Maybe they use different triangle codes for DS games than Wii and Gamecube? Man this is weird

Tom Clancy's Rainbow Six Lockdown has a violet triangle and the languages in the manual are French, Dutch, Italian, Spanish, German and English.
Project Rub (aka Feel the Magic XY/XX) also has a violet triangle but has only French, German and Dutch in the manual.
Well there you have it, the only thing that seems to be set in stone is that dark blue means German language only and dark green is English language only.