How to set the default for a color parameter in a shaderdef?

[Stefano Jannuzzo]

Hi folks.
In a shaderdef define function, I can create a color param and set its
default value to a mid gray by

paramOptions = XSIFactory.CreateShaderParamDefOptions();
paramOptions.SetDefaultValue(0.5);
params.AddParamDef("color", siShaderDataTypeColor3, paramOptions);

Any idea how to set each channel independently?
Thanks. Stefano