YOU MADE ME MAD!
I got it finally
2+1 = 3
4+2 = 6
8+4 = 12
finally...
To be honest this one got me a little mad as well.
It was easy, lots of clues, for a sec. I thought I have mixed some stuff up and wasted everybody's time...
Luckily, everything was OK and puzzle was solvable.
Additional yobicode prize of 750D was sent @yeswepump.
Guess that proper explanation is needed here:
2,4,* and 8(black circles) are hints as well. They show how the chain progresses.
Factor of 2 is the key here. 15 and 4th digit in the code (starting after blue yobit) is number 3.
3 is the first missing digit and then we start multiplying by the factor of 2.
3x2=6 and 6x2=12. That's why the "*" is there, because of nr 12