In ChartDirector 5.0, the colors and the numeric scale are always independent. For this effect you need, you may first a color gradient. For example, you can use Windows Paint or a graphics editor to open your attached chart image, then measure the colors in a few equally spaced points along the vertical color axis. This provides a list of colors that you can use to create the gradient using ColorAxis.setColorGradient.
int colors = ... a list of colors ...;
You can should be able to reproduce any gradient by using sufficiently detail list of colors.
One you get the correct colors, you can recreate the scale. For your case, the scale is:
ColorAxis a = layer.colorAxis();
// axis is from 0.3 to 3
a.setLinearScale(0.3, 3, Chart.NoValue);
// labels at 0.3, 0.9. 2. 3