I do CAD full time at work on Windoze machines, but this Qcad thing isn't the best product for CAD on linux. Use a program called "VariCAD"
www.varicad.com. Don't be fooled by it's title. I know it says it's for mechanical design, well it is but you can obviously use it for anything....in the end, it's all just lines :-) I use VariCad for my side business at home. It's easy to use, and you can create 3d objects very easily. And the answer to your question, is pick the "lightest colour possible - a very light grey". I don't know if you are going to print these, but on the monitor, it will usually give the impression of being "Screened" back. Same as when you print, but it will be more obvious (if you print in grey scale).
If you need an example, let me know. I can send you a Varicad drawing that you can open in your trial version you downloaded.