What determines which parts of the playermodel are colorized by the player's color?

There’s a playermodel I want to edit to make certain parts colorizable, but for the life of me I can’t find any information on how playermodel creators choose this.
I looked in the material, but I didn’t find anything on it so I’m assuming it’s something built into the .mdl file.

If I recall correctly, it’s the alpha channel of the basetexture, where black is not colored and white is colored, and a special vmt keyvalue.

Thanks!

Edit:
Here are the vmt key values:
“$blendtintbybasealpha” “1” This one controls whether or not the texture is tinted based on the base texture’s alpha channel.
“$blendtintcoloroverbase” “0” This one controls how much tinting is done vs how much color replacement is done. 0 means all tinting so use 0 for a colorize effect.

Here is the link where I got this info:
https://developer.valvesoftware.com/wiki/$blendtintbybasealpha

EDIT2: I believe the vmt needs to contain:



Proxies
{
	PlayerColor
	{
		resultVar	$color2
		default		1 1 1
	}
}