The componentWillReceiveProps on RaisedButton makes the zDepth transition based on nextProps, while all other styles are set based on the values of the current props. e.g., here (called from within componentWillReceiveProps > getStyles)

As a result, when the disabled prop goes from false to true, the button has its z-index set to 1 but the color remains grey. Similarly, when disabled goes from true to false, the button gets a z-index of 0 and the default (or primary or secondary) color.