Yep, thems the colours. A3 and C2 are natural, B4 and C3 are synthetic. Germans used motor method for octane rating so C2 and C3 are actually 96 octane, research octane is higher and the sum of the two are divided for pump grade, which means they relate to the first number given in Allied fuels (ie. the 100 in 100/130 avgas for example).
Due to certain additives the German 96 octane performs similarly to 100/130 in the early war and 100/150 by war's end, although the octane ratings don't change for C3 (you'd have to run a captured 109K with the DC motor on 100/150 if you're going to open it right up for example).
On fuel cards for high octane painted to a/c fuselages C2 was sometimes written in the early war (say for an E-4N) whilst 100 was sometimes put on or C3 later, but the actual rating is 96. The fuel card for B4 is always 87 afaik.
AFAIK C2 and C3 are the same colour and rating, but composition is different.
Oh yeah, A-8 invariably used C3 and by that stage it was equivalent to 100/150 according to Crumpp, who has restored an A-8 and can be found around the aviation forums (he's been at LEMB lately).