It just means you hit the hardest (or extremely close to) the max damage you could have done for that hit. Just like regular damage you can hit for example (random numbers for explanation) 500k-1m, a crit max possible damage would be 10m-50m (respectively). A red crit means you hit really close to that number. It is possible to have a yellow 40m crit but a 15m red crit. It is like a little "Hey look how much damage you just did!" kind of thing.
Why are there white, yellow and red damage numbers?
I know white is regular damage and yellow is crits
What is the red one?
Cause I can sometimes hit 150 billion red damage and other times its 800-900 billion red damage
While yellow damage stays around 40-100 million or billion I'm not sure...
It just means you hit the hardest (or extremely close to) the max damage you could have done for that hit. Just like regular damage you can hit for example (random numbers for explanation) 500k-1m, a crit max possible damage would be 10m-50m (respectively). A red crit means you hit really close to that number. It is possible to have a yellow 40m crit but a 15m red crit. It is like a little "Hey look how much damage you just did!" kind of thing.
https://us.battle.net/d3/en/blog/19996041/engineering-diablo-iiis-damage-numbers-1-22-2016
lol @ 150 billion damage