[OS X TeX] Tikz loop question

[Gary Church]

Hello Tikz experts!

I'm trying to do an apparently unusual thing (as I can't figure out  
how to do this :-) )

I've included the "calc" package to get inline mathematical  
operations. What I want to do is the the color of my plots to be  
based of the value of a looping variable. Consider the code fragment  
below:

   \begin{tikzpicture}
     \draw[help lines] (-5,-5) grid (5,15);
     \draw[<->] (-5,0)--(5,0) node[right]  {$x$};
     \draw[<->] (0,-5)--(0,15) node[above] {$y$};
     \foreach \x in {-5,...,-1,1,2,...,5} \draw (\x cm,-2pt)--(\x cm, 
2pt) node[below=4pt] {$\x$};
     \foreach \y in {-5,...,-1,1,2,...,15} \draw (-2pt,\y cm)--(2pt, 
\y cm) node[left=2pt] {$\y$};
     \foreach \w in {1,...,5} \draw[thick,color=black!\w*10,<->] plot 
[domain=-3:3]			%NOTE THIS LINE!
     	function{x**2 - \w} node[right,red] {$y=x^2 - \w$};
     \foreach \w in {1,...,5} \draw[thick,<->] plot[domain=-3:3]  
function{x**2 + \w}
     node[right] {$y=x^2 + \w$};
     \draw[thick,<->] plot[domain=-3:3] function{x**2}
     node[right] {$y=x^2$};
   \end{tikzpicture}

On the indicated line with the comment you'll see "color = black! 
\w*10". My intention is that the curves whose equations are:

y = x^2 - 1, y = x^2 - 2, ..., y = x^2 - 5

are drawn in: black!10, black!20, ..., black!50 respectively.

This doesn't work as I get the error messages:

|147 error| Illegal unit of measure (pt inserted). * ...**2 - \w} node 
[right,red] {$y=x^2 - \w$};
|147 error| Illegal unit of measure (pt inserted).
|147 error| Illegal unit of measure (pt inserted).
|147 error| Illegal unit of measure (pt inserted).
|147 error| Illegal unit of measure (pt inserted).
|147 error| Illegal unit of measure (pt inserted).
|147 error| Illegal unit of measure (pt inserted).
|147 error| Illegal unit of measure (pt inserted).
|147 error| Illegal unit of measure (pt inserted).
|147 error| Illegal unit of measure (pt inserted).

Is what I'm trying to do possible or am I trying to be overly clever?

Thanks much,
Gary.

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