In Magic: The Gathering, how does Ugin, the Ineffable's static ability interact with colorless spells that have X in their cost?
Yes, you can choose 5 for X then only pay 3 mana with Ugin, the Ineffable out.

This is because the process of casting a spell works like this:


Make choices your spell offers you, including choosing modes and the value you'll pay for X in the casting cost where applicable.
Choose targets.
Determine the total cost of the spell, which is its cost plus any cost increases and minus any cost reductions.
Pay that cost.


(There's some other details I've glossed over, but those are the most commonly encountered parts.)

This means in step 1, you're choosing 5 as the value for {X}. Then in step 3, you determine the cost: that's {5} reduced by {2} to get a total cost of {3}.

This also means if you chose 2 as the value for X, you may cast Ugin's Conjurant for {0}.