Another way is to rely on the fact that if the same attribute is written twice, the last one wins. So:
<xsl:template match="/Library/Book">
<xsl:copy>
<xsl:copy-of select="@*"/>
<xsl:attribute name="pub-date">1-1-1976</xsl:attribute>
<xsl:attribute name="color">blue</xsl:attribute>
<xsl:apply-templates/>
</xsl:copy>
</xsl:template>
(Are you quite sure you want to be using the ambiguous date format 1-1-1976?)