xslt variable a list of elements

[codecus]

I have program as follows, but color selection does not work. Does anyone know how to fix this?
fill="{($series_color)/color[$index+1]} this line seems not working, I got fill="" in Firefox and Chrome browser.

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    <xsl:variable name="series_color">
        <color>#99FF99</color>
        <color>#0099FF</color>
        <color>#FF6600</color>
        <color>#990099</color>
        <color>#FF99FF</color>
        <color>#666600</color>
        <color>#CC99F0</color>
        <color>#CC990F</color>
        <color>#CC09FF</color>
        <color>#0C99FF</color>
        <color>#C599FF</color>
        <color>#CC59FF</color>
        <color>#CC95FF</color>
        <color>#CC995F</color>
    </xsl:variable>
 
    <xsl:template name="legend">
        <xsl:variable name="width" select="100 div count(./series)"/>
        <xsl:for-each select="./series">
            <xsl:variable name="index" select="count(preceding::series)"/>
            <rect x="{$index * $width}%" y="0" width="{$width}%" height="100%" fill="{($series_color)/color[$index+1]}" stroke="black" stroke-width="2"/>
        </xsl:for-each>
    </xsl:template>

[Dormilich]

I think it tries to do math rather than construct an XPath.