It seems to me that your issue is one of contract law, rather than strata law. You entered into a contract to purchase the apartment and specified as a special condition that the concrete remain unpainted. There may be other terms in the contract favouring the developer or there may not.
There may be a range of remedies available to you including specific performance and/or damages, but it is likely you will have to to take legal action to get a resolution, or at least start legal action. This costs money and takes time. Of course the developer is stalling, that is what they do. If you don’t want to rescind, don’t, but you will likely have some angst in settling the matter.
Your lawyer is in the best position to advise you. None of the above is legal advice.