Hey Partha!
Yes, all of the tickmarks are passed to the delegate just before they are added to the chart. You can use this opportunity to make any changes you like directly to the label itself:
-(void)sChart:(ShinobiChart *)chart alterTickMark:(SChartTickMark *)tickMark beforeAddingToAxis:(SChartAxis *)axis;
The label is a property on the SChartTickMark object passed to the method - simply update the textColor on the lowest and highest 