Lines are used to help seperate text, images, and tables etc. It helps to organise the page into a more readable fashion. You are able to give it shading or color, but not both.
Splash! allows lines to be any width you like, the width is normally based on the page size used. If the width of the page is 640 then its no good making the width of the line greater than the page.
This is the thickness (or height) of the line.
The line can be of any color that you wish, this allows you to change the color if it is hard to see on a particular colour background.
This allows shading to be applied to the line. However you can not combine both colouring and shading.