<div <?php post_class(); ?>>
post 1
<div>
<div <?php post_class(); ?>>
post 2
<div>
<div <?php post_class(); ?>>
post 3
<div>
<div class="post gauthor">
post 1
<div>
<div class="post gauthor">
post 2
<div>
<div class="post gauthor">
post 3
<div>
.post{
color: grey;
}
.post.gauthor{
color: red;
}