Yes, because the damage is "bonus damage."
Let's take a look at Colossus Slayer and Hunter's Mark, the ancillary damage-dealers in question:
Colossus Slayer: Your tenacity can wear down
the most potent foes. When you hit a creature with a
weapon attack, the creature takes an extra 1d8 damage
if it’s below its hit point maximum. You can deal this
extra damage only once per turn.
Hunter's Mark: Until the spell ends,
you deal an extra 1d6 damage to the target whenever
you hit it with a weapon attack.
Well that's no help, it doesn't even say what kind of damage it is! Let's keep digging. Another ancillary damage-dealer that does keep the same damage type is the Barbarian's Rage feature. It reads as follows:
When you make a melee weapon attack using
Strength, you gain a bonus to the damage roll that
increases as you gain levels as a barbarian, as shown
in the Rage Damage column of the Barbarian table.
I get that one is a damage die increase and the other is a flat damage increase. But they are still both bonus damage sources with no specification. So that makes sense that it's the same type. By this logic, and by Mike Mearls' confirmation of this ruling, the damage types will be the same as their original source. Ergo, if you had a bow that shot arrows of light (dealing radiant damage), the bonus damage would then also be radiant damage.
Now the real question, are they also resisted?
If a creature or an object has resistance to a damage
type, damage of that type is halved against it.
Since bonus damage sources are the same damage type as their original source, then it also stands to reason that they would be halved by the resistance of a foe.
