Ok, well your bricks are red in colour correct? Thus it follows logic that most of your debris would also be red in colour. I spray it down with various dark reds and browns, then paint like your models, ie, washing, drybrushing, dusting, pastels, etc...
Even if all the debris is added, you can still go and get some gutter dust/debris, and add it in, the colours will be just right.
Check out this thread; it helped me a lot when I did this for the first time:
http://www.armorama.com/modules.php?op=modload&name=Sections&file=index&req=viewarticle&artid=690
The bottom two will just give you an idea of some extreme debris in a dio:
http://www.armorama.com/modules.php?op=modload&name=Sections&file=index&req=viewarticle&artid=2150
http://www.armorama.com/modules.php?op=modload&name=Sections&file=index&req=viewarticle&artid=719