[roman]

Intenta con el evento OnAdvancedCustomDrawButton:

Código Delphi [-]
procedure TForm1.ToolBar1AdvancedCustomDrawButton(Sender: TToolBar;
  Button: TToolButton; State: TCustomDrawState; Stage: TCustomDrawStage;
  var Flags: TTBCustomDrawFlags; var DefaultDraw: Boolean);
begin
  if (Button.Index = 1) and (Stage = cdPrepaint) then
  begin
    Toolbar1.Canvas.Brush.Color := clGreen;
    Toolbar1.Canvas.FillRect(Button.BoundsRect);
  end;
end;

// Saludos