Lately I have been working on painting/upgrading a Black Legion army owned by a friend of mine, and part of the upgrade is taking the models I built a few years ago out of bitz from my bitz box to be renegade space marines, and creating a look more in line with them being a traitor legion, instead of more recent traitors. I am trying to do this with as little investment in additional bitz as possible, since that is out of my budget for the project, and will mean that the ambitious project will take even longer to complete.
My solution is to use liquid green stuff to paint/sculpt the appropriate trim, obliterate imperial iconography, etc. So far the first models to receive the treatment are curing right now, but it should only take a couple of layers to build the trim up to the right thickness. The two advantages of using the liquid green stuff are that it is easier to ensure that you do not use too much, and the liquid consistency means that the trim will have a more organic look, which is good, especially with the newer chaos models having a more mutated, organic look to the armour and trim.
Basically the process is this: use liquid green stuff to "paint" the design you want to add to the model, and build up layers until it has the shape and thickness you want, on later layers, wetting your tool helps to make the trim smoother. The technique takes more patience than anything else,but a steady painting hand helps. To apply the green stuff, use either a toothpick, or the awl shaped sculpting tool tht comes with most sets of sculpting tools.