I think it would be useful to have different colors for smite depending on the team the jungler is on. Sometimes it's hard to tell who landed the smite when spectating or playing. I don't know if they are already planning on implementing something like this, but it just makes sense to me.
The preseason jungle changes have 4 different smite colors, 1 for each item (the items are inspired by the aspects of jungling)- ganking(you can smite enemy champs [ehm hem TEEMO] to slow and deal some true damage) dueling (you can smite other enemy champs to reveal and mark them with a debuff that deals true damage when attacking them) counterjungling( gives you increased gold when using smite on enemy camps) and farming(AoE smite damage, stunning creeps). I realise your point though, it does get a bit confusing for players.
Riot buff clarity
Or add some visual effects to the player who casts it.
Before the summoner spells got a VU, they used to leave a sort of aura of light over the player whenever they were cast. This was very noticeable when a player casted flash or smite.
s5 new smite skins lol
If they get the killing blow with Smite, you can already tell by looking in the upper right of your screen.
