In order to make livery creation easier, it would be nice to be able to edit the liveries supplied with locomotives when you purchase them. For example, I'd like to make the GWR Welshman HST power cars, but in order to do that I first have to re-create the GWR livery. It would be nice to be able to edit over the existing liveries instead of starting with a completely blank canvas.
I would love to so as well with my favorite trains. The problem is that copyright protects that. A permission by the copyright owner allows you to use the content as it is, but you're not allowed to modify it. So it's prohibited for DTG. However, I don't know in which way there could be a legal gap for DTG to take to let the users themselves modify it. But I guess it counts as providing the tools to make it. Don't know if you could make a livery mod of the original one in the PC editor though.