Working with collision mesh in NifSkope I have noticed that the wire mesh is sometimes in various colors such as red, yellow and blue.

My guess is that red is normal "bump it" and it moves.

Yellow is that it detects something is hitting it. Sometimes to prevent the player from walking thru .

What is blue?

Red is static non-movable. You can interact with it though.

Yellow is generally only used on weapons and shields.

Blue is usually for clutter items. Anything that can be moved.

There's also purple, which shows up on animated objects.

I've seen green and white as well, but I forget on what. I don't think whatever those are for is something Oblivion is meant to support.

Addendum:

Light Blue: Non-collidable, used on plants for example so you can activate them without the need for a solid collision you can bump into.

White: Transparent, which means you can shoot projectiles through it, but you can't walk through it.

Green: Biped, used for the collision on skeletons. The special thing about it is probably that it acts like a clutter item and non-collidable in combination - not sure though.

Well thank you from me too.
I think I'll just copy and paste this info so when I get back shaking my fist against the screen while using Nifscope and Blender I will atleast know what the colours mean in NifScope

Cheers!

Just shaking your fist? Boy you must have a lot more patience than most

There's also some purple or magenta on animated doors.

I thought that different colours mean different type of collision but meaning box, triangle mesh, etc. My red static collision became purple when I was changing stats in Nifskope to give it animation.

I've also seen yellow collision on rocks, with the OL_TERRAIN layer set. So yellow isn't just for weapons & shields.