There is a color rarity list in the last link
here.
<table width="92%" border=1 align=center cellpadding=3 cellspacing=0>
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<td width="16%"><img src="http://images.neopets.com/items/bluetradingcardback.gif" width="80" height="80"></td>
<td bgcolor="#CCCCFF" width="37%">Blue backs are <b>Very Common</b>, these make up the majority
of cards available.<p>Estimated Price : <b>100-300 NP</b></td>
<td width="14%"><img src="http://images.neopets.com/items/purpletradingcardback.gif" width="80" height="80"></td>
<td bgcolor="#CCCCFF" width="33%"><b>Rare</b> or Purple backed cards are very
hard to find. <p>Estimated Price : <b>2,000-5,000 NP</b></td>
</tr>
<tr>
<td width="16%"><img src="http://images.neopets.com/items/redtradingcardback.gif" width="80" height="80"></td>
<td bgcolor="#CCCCFF" width="37%"><b>Common</b> or Red backed cards are slightly
harder to come by than the Blue.<p>Estimated Price : <b>300-500 NP</b></td>
<td width="14%"><img src="http://images.neopets.com/items/blacktradingcardback.gif" width="80" height="80"></td>
<td bgcolor="#CCCCFF" width="33%">Black backed cards are <b>Very Rare</b>. Be
prepared to search a long time or pay a lot for these.<p>Estimated Price : <b>10,000-20,000 NP</b></td>
</tr>
<tr>
<td width="16%"><img src="http://images.neopets.com/items/pinktradingcardback.gif" width="80" height="80"></td>
<td bgcolor="#CCCCFF" width="37%"><b>Uncommon</b> or Pink backed cards are the
next rarity up from Red or Blue cards.<p>Estimated Price : <b>500-1,000 NP</b></td>
<td width="14%"><img src="http://images.neopets.com/items/goldtradingcardback.gif" width="80" height="80"></td>
<td bgcolor="#CCCCFF" width="33%"><b>Amazingly Rare</b>, these cards are
very, very special. You are fortunate to own a Gold backed card!<p>Estimated Price : <b>30,000-50,000 NP</b></td>
</tr>
<tr>
<td width="16%"><img src="http://images.neopets.com/items/greentradingcardback.gif" width="80" height="80"></td>
<td bgcolor="#CCCCFF" width="37%">Green backed cards are <b>Quite Rare</b>.
<p>Estimated Price : <b>1,000-2,000 NP</b></td>
<td width="16%"><img src="http://images.neopets.com/items/holotradingcardback.gif" width="80" height="80"></td>
<td bgcolor="#CCCCFF" width="37%"><b>Out of Print</b> cards have a <b>Holographic Back</b> - these will
never again be sold in shops!!! Hang on to them!!<p>Estimated Price : <b>100,000+ NP</b>
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</table>
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(I don't know why there is a huge space, but that's how it copied over. lol)