XML : why my status bar color not show?

i cant show work_flow colors in my status_bar ?. anyone can help me?. this my code

           <header `cool_span`="10">                      <button name="signal_draft" type="work_flow"                              string="Reject"                              class="o_e_highlight"                              states="confirmed"                              groups="estate.group_assistant"/>                        <button name="signal_confirm" type="work_flow"                              string="Confirmed"                              class="o_e_highlight"                              states="draft"                              groups="estate.group_assistant" />                        <button name="signal_done" type="work_flow"                              string="Approved"                              class="o_e_highlight"                              states="confirmed"                              groups="estate.group_manager"/>                      <field name="state" widget="status_bar"                             status_bar_colors='{"Draft" :"blue","Confirm":"blue","Done":"blue"}'/>    

please help me to show my widget color