I don't disagree with the almost unanimous sentiments here.
But let's look at this a bit closer, assuming a great paint job, with your preferred colours. If you were to do this during a rental vacancy, you would expect an increase in both valuation and rental. You would also be able to claim 2.5% depreciation for the paint. Capital outlay for an increase in yield. Let's say you spent $1000 on painting and your rent could be increased by $20pw. Payback in one year with some small change depreciation benefits. The increase in value for internal paint job, with nothing else, maybe $5000, if that, but is that of real benefit for a reval? No
Your tenants do an equally great job and your yield actually reduces based on their offer to you. Again, maybe value increases, but whats' the real benefit to you, other than knowing its been repainted.
If you do want them to do it, and they are a painter, you would setup an agreement, having them have the requisite insurances, qualify standards of finish and payment (as in reduced rent) only after your approval of the works have been completed in a workmanlike manner. If they are not a painter, then I would say, would you hire someone to paint your house who has not painted. Really, its messy.
If you want it painted and updated, wait until they vacate and do it yourself or get a handy person/painter to do it.