Kingdom of Knuffel

Right, right, so I've been thinking.

Sound is vibration transmitted through matter, but, mostly, transmitted through air, right?

So obviously the closest element to sound would be air.

So you're an Air!

Hard, let's go with light since you're using a lot of rainbows

You're light, specifically moonlight shining in the darkness.

Undoubtedly air!

You look like you straddle the boundary between light and darkness so well, that I have to go with fire, which in my mind is the boundary between them.... a fire on a dark night provides the brightest light....

Light refracted makes a rainbow, so you're light. :)

... no matter how long I look at it, I really can't see you as anything but fire.

Now, if we can say dual-element, then I would change the answer to a dual-element Light & Darkness, without any hesitation!

This is a toughie, but I think I'll go with air, between the dragon and the rainbows, though I know that's more from water droplets, but still.

I've been hesitating but I think you're light

You belong to the soft serenity of a peaceful night, and the healing powers of slumber.
I think that means Darkness, right?