I understand that you are trying to change the color of the plot.
Below is a sample code for demonstration purposes, which might be helpful in solving this issue:
data = normrnd(3,0.005,100,1);
S = capability(data,[2.99 3.01])
myPlotAxes = gca
child = myPlotAxes.Children(2)
If this does not resolve your issue, please create a Technical Support Request by visiting the link below: