Default values can be set in Thing and Linedef argument definitions.
Example:
9038
{
title = "ColorSetter";
arg0
{
title = "Red";
default = 255;
}
arg1
{
title = "Green";
default = 255;
}
arg2
{
title = "Blue";
default = 255;
}
arg3
{
title = "Desaturation";
}
}