In Jagged Alliance 2: Unfinished Business when you expend additional AP to aim at the enemy, on the final click before you cycle back to no extra AP the aiming circle will turn yellow without increasing the number of AP the shot will take. For example: 6 -> 7 -> 8 -> 9 -> 10 -> 10 (yellow) -> 6. The game manual only says that 4 AP is the maximum you can spend on a shot. What does the yellow circle mean?
Confused me a lot when I played the game the first time, too.
This just means, that you cannot aim better and any further aiming attempt will just reset to the first step. I think it was implemented as some kind of safety measure, so that you know when your cannot spend more APs on this shot. (that you dont have to click 5 times again after reset on accident).
There is no other effect on this yellow circle although I always was kind of superstitious that it "might" have an effect and tended to click to yellow rather than let it stay white.
The manual and even the "official strategy guide" (at least for vanilla) were pretty unaccurate as they were released, it seems that there have been some last time balancing additions to the game that didn't make it to the handbook and guide.