Purple dots and first leaves yellowing

I'm curious to see if the problem moves higher up the plant over time (mobile nutrient) and those dots don't appear to be associated with the plants structure. Is it just one leaf? Does new growth appear normal?

Depending how old the plant is, and how rich the soil is, it looks like it's time to start feeding nutrients. Same thing happened to me with one of my plants and since I started feeding it, the plant is thriving again. Plants will start eating the older, bottom leaves and suck out any remaining nutrients to try and keep growing. I would start feeding nutrients/fertilizer at least once a week and cut away any dead yellow leaves so the plant doesn't waste time and energy trying to repair it. Hope this helps. Cheers!