Glad you solved your issue. Let me point out why it was happening which might help you with other Construct2 instance handling.
Construct2 relies on instances (objects of the same type) being "picked" by conditions so it knows exactly which object you want manipulated.
If you don't let it know which object to pick it will pick all that apply and you will get "fish schooling" behavior.
I highly recommend searching for object picking in the Construct2 manual, very handy info.
Each time you told your car to change colors it changed them all since they are all car_to_choose_2.
Your fix method worked because it basically turned off all but new instances.
Another method would be make an instance variable in car_to_choose_2 called something like colorChoosen=0 and then when it is colored change it to 1 (colorChoosen=1) and condition to colorChoosen=0 (or not (X) colorChoosen=1) in your color change event.
action - car_to_choose_2 set frame random(0,3)
action - colorChoosen=1