Kaffir Lime yellowing

Kaffir Lime yellowing

My kaffir lime tree (more a sapling) is yellowing on the top leaves. It's been pretty cold here lately. The plant is in a large pot. I water and fertilise it regularly. There were a few aphids (or some little black bug) on the top leaves recently but I treated them and they promptly died. Any ideas what could be causing the yellowing? And how I can rectify it.
“Baby Toes”

“Baby Toes”

Gardens in the rain

Gardens in the rain

Tree ID?

Tree ID?

Asian Ribs

Asian Ribs

The rest of my apartment

The rest of my apartment

Just One

Just One