Colossus sig sweet spot

There are 2 factors to take into consideration here. Both the armor conversion from immunities and armor conversion from expiring armor buffs.
My focus is mostly on the second conversion since it is used more often.
At sig 1 it is 25% from immunities and 18% from expiring.
At sig 200 it is 52% from immunities and 45% from expiring.

For me he starts to convert them reliably at sig 80, 41,3% immunity and 34.26% expiring. Which means you take roughly 1/3 of all your expiring buffs. After 80 it slows down a lot. Since there is only 11% to gain in 120 sig levels.
So if you want to save on some stones, I would take him up to 80. Or 100 as @qartweli stated. Both are roughly the same.

HOWEVER! sig 200 makes him more reliable since you almost convert half of all armors and immunities.

Moot4Life

sig 1 is sweet, but 200 is sweeter.
thats for nearly EVERY champion if you think about it
CapIW's sig ability is useless at sig 1. Same goes for Silver Surfer

thats why i said NEARLY