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.
My friend gave me a tree…

My friend gave me a tree…

Temptation!

Temptation!

Intro and a few questions

Intro and a few questions

Succulent ID

Succulent ID

Just an update

Just an update

smile like a flash

smile like a flash