> Also, I don't really know what I am doing, but taking the abs of each
> component separately seems wrong. Rather, a color should be considered
> "bad" if any of R, G or B are outside 0..1 range.

That is what is happening.

Look at this function, if C is outside the range 0...1 then the result 
will be positive, or "bad". Negative is a "good" output.

#declare fD = function(C) { abs(C-0.5)-0.5 }

Now consider this function, feeding into it three results (some could be 
"bad", some could be "good"):

#declare fDist = function(Dr,Dg,Db) { max(Dr,Dg,Db) }

Here "max" is working like a boolean "or" function. It will return "bad" 
(ie positive) if any of the three parameters are "bad".

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