The best way in my opinion to pass the custom class, is in the include of button.tpl:
Use a custom ButtonGrey class:
Code:
{include file="buttons/button.tpl" customclass="ButtonGrey" ... }
Use the default class -
Code:
{include file="buttons/button.tpl" ... }
Then in your buttons/button.tpl:
Code:
<table border="0" cellspacing="0" cellpadding="0" onclick="{$js_link}" class="{$customclass|default:"ButtonRed"}" {if $title ne ''} title="{$title}"{/if}>
<td nowrap> {$button_title} </td>
</table>