It does not change the color of the source, so that things that trigger on a red spell doing damage will still trigger. The damage itself thinks it came from a colorless source, however.

Does not make red and black spells and permanents colorless. They still have color. A red spell can’t target a creature with Protection from Red due to this. The spells just act like colorless sources when dealing damage.

If the source has more than one color but at least one is red or black, then the damage is colorless and all the other colors are forgotten.

The effect is continuous and applies whenever something looks at the damage. If this card leaves the battlefield, damage from red and black spells will appear as its normal color. In other words, the color was not removed from the damage, the color just could not be determined while this card is in effect.