Nothing. I would let it be. There shouldn't be anything for you to worry about its not that serious then.

If you primer the wall before painting you will not have to worry about the yellow color "bleeding" thru at all. But, yes yellow and green will make blue.

when a sound wave hits a wall it refracts and loses velocity but carrys on. if there is another wall within its distace then it will refract gain. this can cause an echo

You must calm them down by telling them that they are gonna get thruogh this and a bullet can't go thruogh the wall with out losing alot of speed no worry.

You should always provide plenty of ventilation for your computer.If you push it against a wall it's likely to overheat.