Activated abilities contain a colon. They're generally written "[Cost]: [Effect]." Some keywords are activated abilities and will have colons in their reminder text.

The extra mana is not considered part of the activation cost. It is a separate cost.

The additional mana is paid at the same time the spell/ability is cast/activated.

The extra mana is part of the total cost, so cost reducers can be applied to this cost.

Gloom : règles, fonctionnement et interactions (MTG)

Gloom is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

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

How it works : Gloom

Original English text — Wizards of the Coast / Scryfall

#1June 8, 2016· Wizards of the Coast
Activated abilities contain a colon. They're generally written "[Cost]: [Effect]." Some keywords are activated abilities and will have colons in their reminder text.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
The extra mana is not considered part of the activation cost. It is a separate cost.
No other card is named in this ruling.
#3October 4, 2004· Wizards of the Coast
The additional mana is paid at the same time the spell/ability is cast/activated.
No other card is named in this ruling.
#4October 4, 2004· Wizards of the Coast
The extra mana is part of the total cost, so cost reducers can be applied to this cost.
No other card is named in this ruling.