as of the status (beta) i think there is no official doc till now.
What i read from source code of beta 3.0 highcharts code, there is no multipoint basis for setting the color.
/**
* Get the zero plane either based on zero or on the min or max value.
* Used in bar and area plots
*/
Found this in source. looks like it just goes with zero OR min OR max. I was looking for a way too with lets say status 'low', 'normal' and 'high'.