Function call on ngClass not working

I have a problem with a ngClass and a function with building an app with Ionic 2 and Firebase. The function does work, but the value of the function does not come in the HTML code. I have used this as an example: https://codepen.io/andre13/pen/oecgi.

The html and the call for the function looks like this:

<button  ion-button class="chatbutton" [ngClass]="compareUser(user.$key)"
(click)="gotoDoctorChat(user.$key)" >{{user.naam}} </button>

The function looks like this:

  compareUser(uid){
this.angFire.database.object('/users/' + uid +'/'+ this.coach1+'red').subscribe(snapshot=>{
  console.log("snapshot.tostring = " + snapshot.$value);
  if(snapshot.$value === '0'){
    return 'unread';
  }else{
    return null;
  }
})

};

The CSS looks like this:

.unread{
    background-color: red!important;
}

Please ask if something is not clear.