>> What do you mean by discarded? A function must always return a value,
>> there is no "discarded" option. Where/how are you going to use these
>> three functions and what do you want the final result to be if the input
>> RGB is out of range? You could return a special value like -1 to
>> indicate a "discarded" vector - but it depends what you're going to do
>> next with the result.
>>
>
> It's for an isosurface, actually, not a color. It just so happens I'm
> modeling a color solid. So color = shape. Sorry.
>
> When any of R, G or B are outside the range 0..1, then I do not want it
> to be part of the shape. I think a select statement can do this.

Ignore all our guesses then as to what you wanted - clipka already 
posted what you should do.

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