Underside of Neo leaves (Kishu Ryokufu) turning reddish

[camille1585]

So this is a very very basic question, but Neos are still new for me. I noticed when watering the Neos this morning that one of the plants (Kishu Ryokufu) has some red/purple coloring appearing on the underside of most of the old leaves. My first guess was too much light, but the newer leaves don't show it, and none of my other Neos have it. I thought it might be related to the variety, but it was perfectly green when I got it early December.

To give you an idea, the plants are on a shelf in a south facing window. Until recently there was zero sun this winter (maybe a few hours a week), but the last 2 weeks have been relatively sunny, and they get direct sun from about 1pm to 5pm. (knowing that the light isn't that strong, the Netherlands is level with southern Canada). I also added a led light strip above them to supplement on cloudy days. As a note, none of my Phals, Paphs or other low light plants show high light symptoms.

Temperature is around 20-22°C during the day, and drops to 16-18°C at night.

I can take a photo later if it helps.