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.
Lurking.

Lurking.

Mystery

Mystery

Need ID on a plant. Imagine that!

Need ID on a plant. Imagine that!

Withered/ dried geraniums?

Withered/ dried geraniums?

Happy New Year!!!

Happy New Year!!!

wishing for more light

wishing for more light