Is there no end to the genius of Jonas Follesoe?
Now he's gone and made an Adobe Kuler plug-in for Expression Blend and Design, making it dead easy to try different colour-ways and create them as resources for your Silverlight or WPF project.
Check it out here: Announcing Colorful Expression Studio Add-In
Then check our Jose Fajardo's post on using Expression Design for comps. I totally agree with him - Design provides a less constrained environment when you are in the conceptualisation phase, plus it can just do some things that Expression Blend can't (including my favourite, text on paths).
Why hadn't it occured to me to put multiple comps on the on canvas though? D'oh!