[kh99]

You don't need array() in the conditional, it should be:

<vb:if condition="is_array($memberpost[userid], $postbuddies)"></vb:if>

But that will only work if you register $postbuddies to the template. A way around that would be to add it to an array that is already registered, like:

$memberpost[postbuddies] = $postbuddies;

then

<vb:if condition="is_array($memberpost[userid], $memberpost[postbuddies])"></vb:if>

I should also mention that rendered posts are cached, so that may also give you trouble. With the post caching, you can't really have the background change based on something that is specific to the user viewing the post.