This could probably be filed as a bug as well, but let's put it here as a suggestion.
Right now the group input of the type "Color" is not interpreted as a texture socket. So I can't plug it into a texture input socket inside the group. However if I plug an RGB color node into the color socket outside the group, it works (see below). It would be great in the color input could be supported as is so that we don't always have to add an extra RGB color node. It's much cleaner and more logical that way.