Elixir luck bonus?

[Snakespeare]

15% chance should mean that after the first roll, there will be a second roll every 7th boss kill. (100/15) = 6.67.

There is no "every 15th... can be a pink". In this case, every 7th has a chance to roll again. Rolling again means another 1.1% chance of getting a pink. So, you have to multiply (0.15 * 1.1) = 0.165. This means that after the first roll giving a 1.1% chance of receiving a pink, there is a 0.165% chance of getting a second roll and receiving a pink.

If the actual number of pinks on re-rolls is greater than 15% times the drop rate, then either the luck description is wrong, or there is a math bug in the game.

No, it is not a "roll again". It is a guaranteed UPGRADE of the color. Futumsh explained it to us. After your roll, it checks the luck. If you get the luck roll you get the next color up. White becomes green, green becomes pink. The only roll left is which green, because there's only one pink.