thanks for playing my game and even record your thoughts :D. I really wish I had the time to explain that mechanic a bit more. After finding out that you need 10 for that pink level, the rest is puzzle solving. I do agree that compared to the other levels, this was a bit of a spike in difficulty.
Essentially the solution is to knock the "-5" away from the side and then go back up around to get the "-3" in to end up with 7 again and win the level.