Extend and soundburst/color spray.

[Lord Ben]

Both these spells are listed as instantaneous, can you extend only the part of them that stuns, not the damage? What's the difference between a spell like daze with a 1rnd duration and a spell like soundburst with an instant duration?

 

[Caliban]

Both these spells are listed as instantaneous, can you extend only the part of them that stuns, not the damage?

No.

What's the difference between a spell like daze with a 1rnd duration and a spell like soundburst with an instant duration?

The duration.

 

[Dr_Rictus]

Both these spells are listed as instantaneous, can you extend only the part of them that stuns, not the damage?

In fact, you couldn't usefully Extend either spell in any way, since they have an instantaneous duration. Twice instantaneous is still instantaneous.

To get an increase to the time spent stunned, you'd need to Maximize or Empower the color spray (since that time is a variable, numeric effect of the spell). And there'd be no recourse with the sound burst, since the stunning effect is not a random variable (although you could, of course, Maximize or Empower it to increase the damage).