When they painted did they ask you for permission to paint and did you give them that permission?
When they changed the light fixtures did you give them permission to change the light fixtures?
How much is their deposit and how much are your charges for painting and redoing some of the electrical problems
How much of the deposit are you refunding them? Will there be any allowances that you are giving to the tenants toward the painting charges since you probably had to paint after they moved even if they had not repainted?
I am sure you have discussed this with your lawyer and what is his opinion? You should consider negotiating the refund with your tenants. Advise them of the cost to repaint over their colors and what part of the cost is their responsibility. If they sue you and it goes to arbitration the court will ask you to negotiate the price anyway.
Take photographs of before and after painting and of the light switches and anything you are going to charge them for.