[RPG] Are Way of the Four Elements monks able to generate the specific element for the sensory effect of Elemental Attunement

The effect lasts only an instant

“Instantaneous” is an adjective that must be attached to “sensory effect” or it stops making any sense as part of that sentence's grammar.

The alternative, not-very-grammatical meaning wouldn't make common-sense sense, anyway: A spell doesn't need to say when an effect happens, when the point of the effect description is by default to say what happens once the spell is cast. Lacking any mention of a delay in a spell's description, the effect is describing the spell's immediate effect.

Thirdly, spells effects are responsible for indicating their duration. If prestidigitation didn't say how long each choice of effect lasted, then they'd be horribly ambiguous and possibly interpreted as being permanent. The “instantaneous” is the require description of the duration of the effect.

So three ways—one a direct reading of the sentence, the next a backup in case grammar is ignored, the third a pragmatic argument that the spell would be broken by ignoring the first two—all indicate very strongly that it is the effect itself that is instantaneous.

Implications for instrument fills

You can't get much in the way of instrument fills out of prestidigitation. At most you can get, as it says, “faint musical notes”, plural, which suggests a trill or other quick run of notes. But also note the “faint” part there: unless you've got a pin-drop-silent auditorium, the faint noises possible with prestidigitation are going to be completely drowned out by other instruments, distance to the audience, even low-level audience noise, or all three.

Prestidigitation is just no good for auditory additions to musical performances.

What about a glow?

A glow doesn't count as a colour, if you're looking at the second-last bullet point and reading “You make a color, a small mark, or a symbol appear on an object or a surface for 1 hour.” That effect option is phrased as a complete list rather than an open list, so the limits of the spell don't allow you to add a glow to an object for an hour.

You can make a glow (on something or on its own) using the first bullet option because that is phrased as general type of effect, with an open list of examples, so a glow is certainly within the realm of a harmless visual effect it can make. But that of course is only instantaneous, not an hour long.

The rule is an extension of a precedent set by WoTC themselves

The lead rules designer of 5e, Jeremy Crawford, has¹ the power to make official rulings, and frequently does so on Twitter, and in the Sage Advice column on the official D&D site. It's common for him to answer questions with some variation of "if a feature was meant to work that way, it would say so." He has even explicitly stated:

Beware of claims that a rule does something mentioned nowhere in that rule or elsewhere in the core books. There aren't secret rules. (source)

Using this principle, he has made rulings such as:

The Dual Wielder feat doesn't include the benefit of the Two-Weapon Fighting feature. It would say so if it did.
(source)

or

If the grease spell created a flammable substance, the spell would say so. It doesn't say so. (source)

From this, we can derive that barring some explicit clarification from Sage Advice, JC himself, an official errata, or a more specific rule mentioned somewhere else in the game's official material, features in the game are intended to only do what they say. Though, of course, the DM is permitted to make their own rulings and allow spells and effects to do things not directly stated in their description as they see fit.

¹ _{While this status was subsequently backed away from (official rulings only now apply to the Sage Advice Compendium as posted by WoTC, not the tweets from J Crawford), it was true that his ruling was authoritative when this question was asked, and answered; that change in state does not change the overall point.}