The model origin represents the (0,0,0) coordinate of the model. When a
sketch is active, a sketch origin represents the (0,0,0) coordinate of the
sketch.
A model origin appears in blue 
and a sketch origin
appears in red 
.
You can add dimensions and relations to the model origin, but not to a sketch
origin.
To change the origin display:
Click
. When the icon next to the menu item is
highlighted, origins are visible except for origins that you hide individually.