The effect is cumulative.

This can lower the cost to zero, but not below zero.

Can never affect the colored part of the cost.

If a spell is both white and black, you pay less, not less.

If this card is sacrificed to pay part of a spell’s cost, the cost reduction still applies.

The generic X cost is still considered generic even if there is a requirement that a specific color be used for it. For example, “only black mana can be spent this way”. This distinction is important for effects which reduce the generic portion of a spell’s cost.

The lower cost is not optional like with some other cost reducers.