1. Anthocyanin pigments are responsible for the red, purple... colors, protecting cells from high-light damage by absorbing blue-green and ultraviolet light, thereby protecting the tissues from photo inhibition or high-light stress.

2. Your plant need food, but; Phosphorus deficiency symptoms include reduced growth and yield, delayed maturity, and generally red or purple coloring along the edge of the plant leaves.

http://ag.arizona.edu/pubs/garden/az1106.pdf

3. A pseudo deficiency created by disease or genetic abnormalities.

4. Pesticide, etc.

IMO

You mentioned it being breezy to windy on the balcony. Is there a ceiling fan or fan that is on in the room where it is now? I ask since the breeze would cool the leaves even in high light. It may be that the light exposure is similar, but the air is still and also touching the glass heating the leaves up more. Just a thought...

There is no fan other than the AC which I turn off during the day so it could be hotter that outside. Thanks, need to get a small fan.

You are welcome. A fan also helps prevent fungal issues

That's sun/heat damage. The redness that people talk about in some catt leaves is more of a burgundy or purple colour, and makes the whole leaf change colour.