I have two seedlings growing in coco right now (link in my sig) and I noticed that they have red/purple stems. I was entirely unconcerned until I thought about it, and I've really noticed anyone posting pictures of seedlings with stems this color. They have been that color since they sprouted. My first thought was P deficiency but they are only 5ish days old which seems early for deficiencies to start. Anyone able to reassure me? Right now I just won't worry about it, and see what happens, but if anyone can fill me in that would be fantastic.
It is completely normal for different strains to show reddish/purplish colors in their stems and petioles when they are young seedlings. Some people believe this to be a way of sexing at an early age. I myself have tested this theory, and I've found like 80% of seedlings that show these colors develop into females. About 15-20% of plants that don't show these colors develop into females. These are just my own finding after sprouting 75-100 seeds. Hope this helps