You can call Axis.update() and pass it an object of Axis config settings. One of those settings can be new plotBands values. So every time you update the gauge value, you reset the plotBands on either side of the gauge value. You'll need to tweak other things to get my example to look just like your image.
Check out this jsFiddle: http://jsfiddle.net/ZrGut/
yAxis.update({
plotBands: [{
from: 0,
to: leftVal,
color: 'pink',
innerRadius: '100%',
outerRadius: '0%'
},{
from: leftVal,
to: 90,
color: 'tan',
innerRadius: '100%',
outerRadius: '0%'
}]
}, false);