Did you try
.post-156 h1{
color: #666666 !important;
}
If one of your wrapper elements, like <article> is unsing post_class() the post ID becomes a class of this wrapper like .post-156. So you can say all h1 which are contained in a wrapper with the class .post-156
Another possibility, if your <body>-Tag is using body_class():
.postid-156 h1{
color: #666666 !important;
}
Hope, this helps.
Ref.: