$desaturatewithbasealpha is a material shader parameter available in all games since for
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
The amount of desaturation is determined through the same parameter.
1.0 will result in 100%, and
0.5 in 50% desaturation.