When a style has no rules, the "else" symbolizers apply. When a style has rules and none of them apply to the given feature, the "else" symbolizers apply. Note that this is different than default symbolizer properties that might be merged into all symbolizers (as in OL2) - I don't think we should support that.
The color names are more intuitive. And if we want to support pattern strokes or fills, we'll need additional proerties to represent other pattern properties.