Which, if any, colored cards can be made colorless after being cast?
No spell has an inherent way to become colorless after it has been cast while it's still on stack. There are ways to do this however, but it requires using other cards. This means that almost all spells can be made colorless, the only ones that can't are spells that can't be targeted or prevent using the stack until they resolve (split second).

Change target spell to colorless:

Ersatz Gnomes is an artifact creature that a tap ability to turn a targeted spell on the stack colorless.
Moonlace is a one cost blue spell that will turn a target spell on the stack colorless.


Permanents that change themselves to colorless. (all red creatures)

Ancient Kavu 2 cost activated ability to turn itself colorless.
Blazing Blade Askari 2 cost activated ability to turn itself colorless.
Raging Spirit 2 cost activated ability to turn itself colorless.


Turns another permanent colorless.

Ersatz Gnomes has a second tap ability to turn a targeted permanent colorless until end of turn.
Moonlace can also target a permanent to turn it colorless. This effect is permanent.
Xathrid Gorgon is a black creature that has a tap ability costing 3 mana that will permanently turn a target creature into a colorless artifact and gives it defender (flavored turn to stone gorgon ability).


Change everything to colorless.

Thran Lens this 2 cost artifact makes all permanents colorless, this applies to all permanents controlled by all players.
Mycosynth Lattice this changes everything to colorless, in play, on the stack, in the hand, in the deck, etc.
Ghostflame Sliver this 2 cost black and red creature doesn't apply to everything, but it applies to all slivers, regardless of who controls them.