Tonight I rolled my first Nat20 during combat, I had hunters mark on the target and I also took Colossus Slayer previously. 

Now I was under the impression that all dice would be doubled or rolled twice, but I was told it was just the base longbow damage. 

So to break it down:

Longbow 1d8 x 2 + Colossus Slayer 1d8 + Hunters mark 1d6

I've read online that Colossus Slayer should also be doubled and I was wondering if someone here could please enlighten me as to what the correct/official way would of been to calculate the crit damage in that situation. 

Thanks in advance for any help <3

Hunter's Mark and Colossus Slayer are both rolled twice on a crit. The rules state that you roll all of the attack's damage dice twice, not just the weapon's.

Basically you want to check if the effect says you add an additional X damage; if that's the case then it's all one block of damage, and any dice are going to be rolled twice with a crit.

Off the top of my head, I can't think of any situations where you wouldn't roll a die twice on a critical hit.

Okay, wait, I have one. There are a couple of rules that add dice only when you crit. Brutal Critical from the Barbarian class, and the spell Blade of Disaster. Maybe others? These rules have been argued to death because it's a little ambiguous on first reading whether you should or shouldn't roll those dice twice. Luckily we don't have to get into it here.

For everything else, yes, roll it twice.

Quote from ChoirOfFire >>

Off the top of my head, I can't think of any situations where you wouldn't roll a die twice on a critical hit.

Okay, wait, I have one. There are a couple of rules that add dice only when you crit. Brutal Critical from the Barbarian class, and the spell Blade of Disaster. Maybe others? These rules have been argued to death because it's a little ambiguous on first reading whether you should or shouldn't roll those dice twice. Luckily we don't have to get into it here.

For everything else, yes, roll it twice.

 
The Half-Orc’s Savage Attacks too.