Editing group node sockets, with no external input? Blender 2.76

Is it possible to edit a group node's inputs by only editing the sockets? Without connecting external nodes to the sockets?

I have created a group node containing two diffuse shaders, connected to a mix shader, that uses a gradient texture as the factor.

I then connect the colours of the diffuse shaders to colour inputs.

If I change the colours in the group's sockets, it doesn't change the colours of the object. It still uses the default colours, which in this case were taken from the colours of the diffuse shaders.

However, if I connect a colour output to the colour socket, those colours overwrite the default colours.

I'd like to edit the colours by changing the colours in the sockets of the group. It seems that, since I can change the colour of the socket, it should affect the shader, shouldn't it? Is this possible?

This gif might help:


And here is the .blend file:
I downloaded Blender 2.76b, opened my .blend file inside it, and it didn't have this problem. Perhaps it was as easy as that?