Gloom

Enchantment

White spells cost more to cast.
Activated abilities of white enchantments cost more to activate.

2004-10-04:
The extra mana is not considered part of the activation cost. It is a separate cost.
2004-10-04:
The additional mana is paid at the same time the spell/ability is cast/activated.
2004-10-04:
The extra mana is part of the total cost, so cost reducers can be applied to this cost.
2016-06-08:
Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keywords are activated abilities and will have colons in their reminder text.