The reddish coloring is probably from cold temps at night or genetics. Are temps nearing or dropping beow 68F? It doesn't take much. some turn "Fall" colors no matter what, too. Might just end up with purple/red leaves. If temp-related, probably best to prevent it, but they seem to still develop fine in my experience. I'd assume it impacts its ability for photosynthesis a little bit.
It sure does look like an N-def, but as others said, it's in the wrong location, which is weird. Looks like it might be worse lower on the plant? if so, that tracks well. Add some N relative to recent levels.