$desaturatewithbasealpha
... 
$desaturatewithbasealpha is a material shader parameter available in all 
Source games since 
Alien Swarm for VertexLitGeneric.
Setting it to a value above 0 will enable the behavior.
If used, the $basetexture's Alpha channel will be used as a mask to de-saturate the $basetexture's Color.
The amount of desaturation is determined through the same parameter. 1.0 will result in 100%, and 0.5 in 50% desaturation.
Todo: Is this available since 
?
Note:Also available for use in 
and 
through 
LUXCaveats
Bug:This Parameter does not work with
$seamless_base. (this works in)$detail. (this works in)$bumpmapor any parameters that requires a$bumpmap. Including$lightwarptexture. (this works in)$selfillumand thus $selfillummask (this works in)$selfillum_envmapmask_alpha. (this works in)$basealphaenvmapmask. (this works in)
Warning:Using the Basetexture's Alpha channel for other effects at the same time might result in unexpected behavior!