<table>
    <caption> Table 1</caption>
    <tr>
        <th>Table Head 1</th>
        <td>Table Data 1</td>
    </tr>
    <tr>
        <th>Table Head 2</th>
        <td>Table Data 2</td>
    </tr>
    <tr>
        <th>Table Head 3</th>
        <td>Table Data 3</td>
    </tr>
</table>

<table>
    <caption> Table 2</caption>
    <tr>
        <th>Table Head 1</th>
        <td>Table Data 1</td>
    </tr>
    <tr>
        <th>Table Head 2</th>
        <td>Table Data 2</td>
    </tr>
    <tr>
        <th>Table Head 3</th>
        <td>Table Data 3</td>
    </tr>
</table>

<table>
    <caption> Table 3</caption>
    <tr>
        <th>Table Head 1</th>
        <td>Table Data 1</td>
    </tr>
    <tr>
        <th>Table Head 2</th>
        <td>Table Data 2</td>
    </tr>
    <tr>
        <th>Table Head 3</th>
        <td>Table Data 3</td>
    </tr>
</table>​

$('table').find('tr:even').css('background','#d0d0d0');

$("table").find("tr:even").css("background", "#d0d0d0");

$('table tr:nth-child(2n)').css('background','#d0d0d0');

$('table').each(function(){
   $(this).find('tr').filter(':even').css('background','#d0d0d0');
});

<table id="t1">
    <caption> Table 1</caption>
    <tr>
        <th>Table Head 1</th>
        <td>Table Data 1</td>
    </tr>
    <tr>
        <th>Table Head 2</th>
        <td>Table Data 2</td>
    </tr>
    <tr>
        <th>Table Head 3</th>
        <td>Table Data 3</td>
    </tr>
</table>

<table id="t2">
    <caption> Table 2</caption>
    <tr>
        <th>Table Head 1</th>
        <td>Table Data 1</td>
    </tr>
    <tr>
        <th>Table Head 2</th>
        <td>Table Data 2</td>
    </tr>
    <tr>
        <th>Table Head 3</th>
        <td>Table Data 3</td>
    </tr>
</table>

<table id="t3">
    <caption> Table 3</caption>
    <tr>
        <th>Table Head 1</th>
        <td>Table Data 1</td>
    </tr>
    <tr>
        <th>Table Head 2</th>
        <td>Table Data 2</td>
    </tr>
    <tr>
        <th>Table Head 3</th>
        <td>Table Data 3</td>
    </tr>
</table>

$(function(){
    $('#t1 tr:even, #t2 tr:even, #t3 tr:even').css('background','#d0d0d0');
});

$('table').find('tr:even').css('background','#d0d0d0');