I know money is tight but surely this is just part and parcel of maintaining the building.
Seems like you have no idea how tight money actually is. The government is not giving schools enough funds to cover even the basics, and yes, that includes basic building maintenance. That's why more than a thousand headteachers marched on Downing Street recently.
When I see posts like yours, it makes me realise that parents just don't get how bad the funding situation has got. As a school governor, it's very clear that the current situation is unsustainable, and I can totally see why asking parents to volunteer for such tasks might seem like a good idea. The alternative is that the walls just won't get painted.
The situation is dire. We are failing our children, and as a country, we need to wake up to it. Otherwise, parents will be doing a whole lot more than just painting.
If you don't think that you should have to volunteer to help out with basic maintenance (and I agree that you shouldn't), then don't do it but lobby your MP about sorting out the funding crisis. Better still, paint the walls AND lobby your MP. The current situation cannot be allowed to continue.