You figured it out when you wrote the question. It's because the engine nacelles are equipped with anti-icing heaters, and painting those areas would be counter-productive. Same with the inboard sections of the wings.

You figured it out when you wrote the question. It's because the engine nacelles are equipped with anti-icing heaters powered by air from a compressor port, and painting those areas would be counter-productive. Same with the inboard sections of the wings.