I figured out the solution, we can set colormap and then imshow to display respective images..
% --- Executes on button press in pushbutton1.
function pushbutton1_Callback(hObject, eventdata, handles)
% hObject handle to pushbutton1 (see GCBO)
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)
[img,cmap] = imread('peppers.png');
cla(handles.axes1,'reset');
colormap(handles.axes1,cmap);
axes(handles.axes1);
imshow(img,[]);
% --- Executes on button press in pushbutton2.
function pushbutton2_Callback(hObject, eventdata, handles)
% hObject handle to pushbutton2 (see GCBO)
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)
% global imB
temp = imread('cameraman.tif');
cla(handles.axes1,'reset');
colormap(handles.axes1,gray);
axes(handles.axes1);
imshow(temp,[])