TwentyFifteen: How can it show a different color on every post page?

The CSS code syntax is not correct. For post and page specific CSS, you need to use something like this:

/* Replace x with post/page id and post with page and vice versa*/ 
.page-id-x .hentry, 
.page-id-x .entry-footer {
   background-color: #RGBColor !important;
}