Since my Grey Knights are predominately dark cool colors, I decided to use a warm light colored base to set off the miniatures. These quick iPhone pictured don't quite do the minis justice, but they do show how contrasting temperatures can help with presentation. I need to be careful going forward to not make the reds on the GK too warm. I can do this by using blue washes to shade the reds rather than red or brown washes.
My bases are dirt simple. I painted them with Army painter bone, and then used the dark earth forge world weathering powder as a wash. Afterwards I went back over them wiping off most of the powder from the raised surfaces. The bases are plain, but I think that helps the dark miniatures stand out.