Change color of polygon outline by map scale

ArcMap Desktop 10.0, Visusal Studio 8.0, vb.net.

I have a taxlot polygon layer. I want to change the color of the polygon outlines after the map refreshes below 1:10,000 mapscale.

Am I looking at an addin extension? I want this to happen automatically with not user input. I have written multiple vb.net addins, all buttons or tools, but no extensions.

Can someone point me in the right direction.

Thanks,

jb