How does Charmed Pendant interact with Reaper King?
You have to add WUBRG to your mana pool.

From the Comprehensive Rules:


  107.4e Hybrid mana symbols are also colored mana symbols. Each one represents a cost that can be paid in one of two ways, as represented by the two halves of the symbol. A hybrid symbol such as {W/U} can be paid with either white or blue mana, and a monocolored hybrid symbol such as {2/B} can be paid with either one black mana or two mana of any type. A hybrid mana symbol is all of its component colors.


Colorless is not a color, so you cannot add colorless mana to your pool in this case.


  105.4. If a player is asked to choose a color, he or she must choose one of the five colors. “Multicolored” is not a color. Neither is “colorless.”


Also note the rules clarification on the card's Gatherer page: 


  If you flip over a hybrid card, you get one of either color mana, not both, for each mana symbol.


This is just in cases where you flip other, "proper" hybrid cards with 2-colored hybrid mana symbols. In this case, you don't even have a choice as you can't pick colorless for a color.