Tenant wants to paint bedroom

Tenancy Types and Management

I would really appreciate some advice from more experienced landlords. My two tenants are 4 months into their second year. They have been great and I'm keen to retain them.
The property was totally refurbished to a very high standard and had a professional paint job less than 18 months ago.
My tenant has asked to paint her bedroom. I am reluctant to agree having seen so many bad DIY jobs, plus it could open the door to unsuitable shades throughout the house. It has a modern interior with white walls and oak floors and doors, and I think it looks great as it is.
The lease says my agreement to changes would not be unreasonably withheld. Would I be unreasonable to say no? They may only stay a few more months. If I agreed to it would it be reasonable for me to ask them to use my decorator who I know will do a good job? And could I ask them to return the room to its original condition before they leave?
Your advice would be much appreciated.