Absolutely nothing wrong with this plant. Many brassicas have a reddish-purplish tinge to their foliage. This can also develop if the weather is cooler than they like, especially at night.

I have brussel sprouts growing here in Florida, and not sure if they will even do anything since the weather is getting hot and humid. Some of the plants are tall and leafy and noticed yesterday the veins turning purple. I saw a video on brussel sprouts when we first planted them and it said to cut the leaves on the bottom working up so that there is one layer of leaves at the top, Well we did that and have no brussel sprouts on this plant. It's very tall and healthy tho.
Then we have one that looks like it is sprouting brussel sprouts but we didn't cut the leaves. Anyone know what might be happening to the taller one. Thanks for any info