CSS use :not ID with CLASS

I believe that you are reversing the selectors. You have some elements with the same class, but you want to filter out an element with an specific ID. In that case:

HTML:

<p class="someclass">hello</p> <!-- will be targeted by css below, thus green -->
<p class="someclass" id="some-id">hi</p> <!-- will not be targeted by css below -->

CSS:

.someclass:not(#some-id){ color: green; } 
/* selects all elements with classname 'someclass', 
   but excludes the one that has also has an id of 'some-id' */

And as @secretSquirrel pointed out, note the browser compatibility: this selector is not supported by Internet Explorer 8 and older.