My pincushion/protea leaves have turned yellow
My research says that some leaf yellowing on these plants is normal. However, your plant may need some liquid sulfur to acidify the soil a bit more. I would pinch off and clean away any dead leaves and debris, then check all your growing conditions against those listed in this article: https://www.gardeningknowhow.com/ornamental/flowers/protea/protea-plant-care-tips.htm