The first thing to do is to clarify that your tenants do not expect payment or a reduction of rent in return for painting a room. Secondly does the room need painting?
If so, are you confident the tenants can do a neat and proper paint job. One approach may be for you to agree on the colour with the tenants and for you to pay for the paint and materials needed.
Often tenants who have been in a property for a number of years will look to paint some rooms.