Wish we had a picture! But, your instincts are probably correct, the purply stems don't sound like tomatoes or peppers. Not sure what they do sound like, I can think of numerous flowers & vegs that start out purply, but *anything* could have been in the compost mix. If you are curious about their identification, let a couple grow until the adult leaves form and maybe you can then determine the interloper's identity. Now you know the second reason for sterilizing seed-starting soil :)

Tomato seedlings often have purple stems and under leaves. It has something to do with potassium intake (not enough) but it is normal and they will outgrow it. Do not pull out your little tomato seedlings!

Nurmey is right, I had forgotten what could happen if a good soil mix isn't used! OTOH, it's probably been more than 30 years since I've seen an off-color tomato stem; but now that the memory returns, I have seen reddish-purply ones, usually on heritage varieties. Sorry, I didn't keep them, so I have no idea how they perform as grown-ups.

The red to purple pigment that develops in leaves/stems is anthocyanin. Production is dependent on how much photosynthate (sugar) is stored in the leaf or stem. Anthocyanin production also increases as phosphate moves out of (particularly young) leaves as a result of cool spring weather (or seedlings being kept at cool temps). Generally speaking, when growth is limited by cultural conditions, but photosynthesis continues, anthocyanin builds up in leaves and/or stems, turning them red/purple.

Al