Changing the torso appearance determines the AC. The toolset lists the torsos in ascending order of AC, so, for a light armour, scroll through the low AC models & choose one with the AC you want.

Other body parts have no affect on AC.

Changing colour doesn't change the torso number in my experience.

Seems broken to me, but ok. It used to be select "light", "medium", "heavy" and then whatever torso appearance you wanted. Now L/M/H armor is attached to the torso look. ok. Clicking on the "color" button changes torso and legs too. Seems broken to me since the most recent patch.

Changing color bugs for me too, but only when using haks.
Works fine without any.

I checked the toolset Item Wizard, and edited the armour template resulting. It doesn't ask me to select light / medium / heavy, nor does changing colour alter the torso number, which makes me wonder what tool the OP is using?

There's no general problem with haks, either. Could it be a custom character editor hak or mod that's bugged?

I'm using the Aurora Toolset Editor. I don't have custom haks installed, aside from what came with the premium content. Toolset editor about window is version vts80, "Game Version" is v1.87, expansion packs installed: Expansion Pack 1, Expansion Pack 2.

I see there is a temporary visual problem in the toolset "Changing the color on torso 010 changes the torso look to 036." It really doesn't matter - just save the armour and open it again, you'll see that the torso is still 010.

I'll report it.

It's a known .35 issue, apparently, with other instances of .plt textures misbehaving in the toolset.