You can't change its color, but you can add a simple string annotation to the point. The line below adds a simiple "XY" annotation to point #23 (numbering starts at 0).
c# this.AddAnnotation(SPCChartObjects.PRIMARY_CHART, 23, "XY");
or
vb Me.AddAnnotation(SPCChartObjects.PRIMARY_CHART, 23, "XY")
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