Click the options icon on the Data->Outline group. This is the same group that holds the Group, Ungroup, Subtotal, etc. actions, but the icon looks like a little square with an arrow coming out of the bottom right corner and is on the bottom right corner of the group (in line with the Outline text).
In the dialog box that comes up, uncheck the 'Summary rows below detail' box and hit OK.
Now, for the Red T-shirt example, highlight all of the 'Red T-shirt' cells EXCEPT for the first one (i.e. highlight B6:B9). Click on Data->Outline->Group and choose Rows.
That should do it.