Hey, I’m trying to figure out how to EDIT models. I want to edit the metro-police model, I want to add color to the shoulders on the uniform. What software would I need to just add two colors on each shoulder. I don’t want it as a skin replacement I want it as a new model so it can be used in for example Half-Life 2 roleplay. Thanks.

You could use Blender, it’s free, easy (as for me it is) and you can download a SMD importer/exporter for it, and there’s lots of tutorials on how to use blender.
Here’s a link to their download page :