Let's say Nelson has an Emrakul, the Aeons Torn, and a Painter's Servant naming red. Nice board state.

Alan casts Ulamog, the Infinite Gyre. He forgets the triggered ability for some reason. After Ulamog resolves, he realizes this and calls for a Judge. The Judge offers Nelson the choice to resolve the trigger as usual, and for some awkward reason, Nelson says “yeah sure whatever”

Since we don't back the game up for missed triggers, are we treating the source of this triggered ability as Ulamog the permanent, or Ulamog the spell? The latter seems to be the most intuitive answer, but Ulamog -isn't- actually a spell anymore so I figured it was worth seeking comment from the peanut gallery. What this question really boils down to is… Can Alan now target Emrakul, or not?

Edited Scott Marshall (June 18, 2013 09:28:52 AM)

This is actually a rules question; locking until we get an official answer.

Sorry for the delay on this.
Interesting question; there's actually two aspects to this.
First, the rules question: does Emrakul's “protection from colored spells” apply to Ulamog's triggered ability? CR 702.15b tells us that protection extends to “abilities from a source with the stated quality”, and the source of that ability is, in fact, a colored spell (thanks to Painter's Servant). So Emrakul is not a legal target for Ulamaog's “when you cast ~this~” ability.
Second, the judging question: if we do put the trigger on the stack after Ulamog has become a permanent, instead of a spell, does that change anything? This gets a little wonky, and there's probably some really odd corners involving game state changes that happen between a trigger firing and a trigger being remembered and put on the stack. However, in general, I think we can look at the philosophy of this sentence:

Originally posted by IPG 2.1:

No player may make choices involving objects that were not in the appropriate zone or zones when the ability should have triggered.

and conclude that the Ulamog trigger, when it's put on the stack later, is still an ability whose source is a colored spell. No targeting Emrakul just because you forgot that trigger and your opponent wants it to happen now.