<% if @user.id == 5 %>
 <div id="bill-franklin">
   CONTENT
<% elsif @user.id == 6 %>
 <div id="joan-smith">
   CONTENT

<div class="<%= @user.color %>" >
  CONTENT
</div>

<div class="my_user_<%= @user.id %>" >
  CONTENT
</div>

.my_user_1 { background-color: red; }
.my_user_2 { background-color: blue; }