To set the colour of specific objects, first load an image and run Segmentation to create an image with associated edges. Select a few edges by clicking on them with the left mouse button. To deselect a line click on it with the middle mouse button.
Then use the Geometry option to select a new colour for the selected edges. The edges will be unselected and displayed in the new colour.
The Geometry menu item can be used to change the width of all selected edges in the image.