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.
Seed starting the thrifty way

Seed starting the thrifty way

Not about any plants but certainly garden related.

Not about any plants but certainly garden related.

I grow younger in heart :D

I grow younger in heart :D

My garden is coming back!

My garden is coming back!

Kitchen accent color ideas?

Kitchen accent color ideas?

A kitchen garden – at last!

A kitchen garden – at last!