From Valve Developer Community
Modulate is a shader that modulates the colors of any pixels behind it.
Tip:This shader can be effectively used to fake various features that Source may not support, such as vertex coloring or lightmapping on models, by creating a surface above the first one to modulate it.
Accepts only the following parameters:
$basetexture "material" //Multiplies the color behind it with this surface's texture. $mod2x "boolean" //Doubles the modulation, making it appear brighter. 1 enables this, 0 disables. Disabled by default. $nocull "boolean" //Draws triangles from both sides, not just one. 1 enables this, 0 disables. Disabled by default.
Note:Does not accept any translucency parameters, including $alpha, $alphatest, $additive and $translucent.