rule of thumb on painting in "open floorplans" - how to define spaces or should we?

[cablevins]

We're buying a new home - it'll come with the whole house all painted the same color. I'd like to change that.

The downstairs is pretty much all open - den with eating area to right, then kitchen back from that - would it be best to paint the whole area the same color....I'm just not sure what to do w/it.

Thanks for any tips!