[jQuery] alternate color row starting from a point

[Magritte]

Hi every body,
i have a table like this (simplify):

<table>
<tr class="className"></tr>
<tr></tr>
<tr></tr>
<tr class="className"></></tr>
<tr></tr>
<tr></tr>
<tr></tr>
<tr class="className"></></tr>
<tr></tr>
<tr></tr>
</table>

I would like to color the blackground of the <tr> without class