Smarty has a built in function for alternating row colors!
<tr bgcolor="{cycle values="#F3F3F3,#FFFFFF"}">
Code:
<table width="400">
{section name=ship_num loop=$shipping}
<tr bgcolor="{cycle values="#F3F3F3,#FFFFFF"}">
<td><label>
<input name="shippingid" type="radio" onSelect="" onClick="document.cartform.submit()" value="{$shipping[ship_num].shippingid}" {if $shipping[ship_num].shippingid eq $cart.shippingid}checked{/if}>
{$shipping[ship_num].shipping|escape}</label><td align="right">{include file="currency.tpl" value=$cart.shipping_cost}</td></td>
</tr>
{/section}
</table>