Tezzeret, Master of the Bridge

Affinity can reduce only generic mana in a spell's cost. It can't reduce color requirements.

Affinity doesn't change the spell's mana value. It changes only how much mana you pay to cast the spell.

Affinity for artifacts means "This spell costs less for each artifact you control."

If a spell has multiple instances of affinity, each applies.

If there are any additional costs or cost increases that would apply to a spell with affinity, apply those before applying any cost reductions.

In a Two-Headed Giant game, Tezzeret's first loyalty ability causes the opposing team to lose twice X life and you gain X life.

Tezzeret himself doesn't have affinity for artifacts.

The cost reduction is set before you pay any of the spell's costs. Specifically, you could lock in a discount for an artifact you control and then sacrifice that artifact to activate a mana ability.