How do I Describe This?

First, I want to say that this reads much better than your previous version.

As for the gesture, maybe:

The guard moved his hand across his chest in a wave like motion.

 

The gesture- The guard looks wildly at the Illusionist and makes a slight waving moment across his chest. "They shift and flow throughout his robe. It is just strange...

 

Hey Dan -
Cool effect for the character.
RE the hand motion - I'd actually suggest keeping it as simple as possible: The guard smoothed a hand down his chest.
Here's why: The description he says right afterwards: "The colors melt and flow like molten rock" is magnificent. It's awesome. Let me take *that* awesome image out of that whole paragraph instead of trying to picture a dude waving his hand all crazy in front of his chest.

HTH,
J

 

Perhaps take it out entirely? As far as clarity, I think the gaurd's description of it does well enough.

Otherwise, maybe something like:

Or...

Or simply...

 

I think this would make for a sufficient beat:

“His robe, sir.” He hesitated. “The colors shift and flow like molten rock.”

 

It's a great line with a great rhythm. This is how I read it:
His ROBE sir. Those COLors. They SHIFT and FLOW like -" he paused, "MOLten rock."

Now here's my issue with how it's punctuated and using the word "paused" as the beat. It's a little heavy-handed, like "just in case the dear reader didn't understand the significance of what's going on, there's 3 markers to make sure."

1. The emdash at the end of the third sentence in that dialogue line already signifies a pause/break/beat of silence/ broken dialogue line.
2. Just in case it's not clear though, there's literally a break in the dialogue line to insert the attribution.
3. Just in case it's really not clear enough that the literal break in the dialogue line and the emdash mean a pause, the word "pause" is in there.

So, three things being used to accomplish what one thing could do.

Why is the guard pausing?

 

Ok, cool. I sort of got that with what you were trying to do.
Now let's take a step back and take a look at the bigger picture.
Plant the guard *in* your world. I feel like, from the previous attempts, you are *you* the writer, trying to describe what the guard is doing.
How about putting yourself *into* the guard - I mean, complete immersion - to see/feel/convey what he might be thinking/feeling.

Here's what I mean.
How it reads now is he's just a normal ol' generic guard lifted out of normal ol' generic fantasy opening.

How is this guard special to *your* world?
I think we can look at his relationship to the main character, the current POV character, to see perhaps what the MC expects to see? Or perhaps, the opposite? Which would let the reader infer half a dozen things about the guard, about the guard's relationship with the MC, the guard's relationship with your unique fantasy world. You can accomplish all this *plus* get the beat you're looking for by exploring and showing what makes your world unique, different, from every other story.

What would the MC notice in particular? What can you reveal about the MC by showing what he understands he sees, versus what else can be generically described?

Frex:
If the guard is young or new or scared of the MC, and perhaps the MC holds all his servants in contempt?-
"His r-robe, sir." The guard made a non-sensical gesture with his trembling hands. "The colors shift and flow like m-molten rock."

Perhaps the guard is an old veteran who couldn't care less about whether the MC understood?
"His robe, sir. The colors shift and melt like..." He smiled crookedly and shrugged. "Like the iridescence of oil on water."

Anyway, HTH
J

 

A lot of useful points have been made here already, I don't really have anything new to add, BUT let me share my initial thought anyway:

You, the writer, don't know what kind of hand gesture to make (describe) to illustrate how the colors move... maybe the guard wouldn't either? I get the sense that this guard in perplexed by the way that robe's fabric behaves... so I'd put that into the narrative.