How to populate users based on their role?

For e.g. I have taken lookup field which consist of all the roles.

Once a role is selected, a picklist should get populate with all the users under that category of role.

                     I wanna know is that.... whenever we login in deveolper editon we come across UI which has Home and other tab, i'm want to change thats backgroud color and make it attractive ... Is this possible? and if this future is not present in developer edition , can get this option when I actually develop application for client(Paid Edition).?

Thnks

san