Hi Dipti, thanks for trying out the app. You can modify the current colormap through Options -> Set Colormap. You can pick from the default colormaps, invert the current colormap, or select Custom and select a variable from the workspace that contains a custom colormap. If you want to reuse a colormap or set it to be your default, use the Options -> Set/Load Options menu in the app.
If you are looking to interactively construct a custom colormap, the easiest way I know of is in the regular Figure menu Edit -> Colormap. See the help text at http://www.mathworks.com/help/releases/R2013b/matlab/ref/colormapeditor.html#f19-785301 for more info.
So you can open up a regular figure window and use the colormap editor to construct the one you want. Once you modify the colormap for the figure, you can grab it with a command like >> cm = get(gcf,'Colormap');
and then import that into the app using Options -> Set Colormap in the app. You can then use the Options -> Save/Load Options menu in the app to save it for later use.
And now this has got me wondering if I could just invoke that colormap editor directly in the app....I'll look into that.