How to send a custom variable into the pixel shader?

I have a new pixel shader that I wish to use to allow the user to modify the color of something from within the game's GUI system. It seems simple enough to pass in a custom variable, however I'm hitting a snag somewhere.

I see a few possibilities, such as utilizing the register at PC_USERDEF1. The process looks like it should be simple enough. There is a float4 value sent into the HLSL from the script, just like the way we currently send in the specular color. So I would want to have the following:

new CustomMaterial(redWood)
{
   mapTo = "redWood_d";
   shader = StandardShader;
   version = 2.0;

   texture[0] = "redWood_d.jpg"; //Diffuse Map
   texture[1] = "redWood_n.png"; //Bump Map 

   specularPower[0] = 10;
   specular[0] = "0.6 0.6 0.6 1";

   customColor = "0.4 0.2 0.6"; //<--------- Custom Color Definition Variable
};

All I need is the ability to pass that 'customColor' variable into the HLSL script. Anyone think they can give me a shove in the right direction on how to pass in custom variables?

Thanks in advance!
-Dave Calabrese
Gaslight Studios