get(h.Parent, 'ColorOrder')
will give you a list of colors and first-row corresponds to the first plot (i.e. your histogram). By default it is
0 0.4470 0.7410 <- This is rgb value for your histogram
0.8500 0.3250 0.0980
0.9290 0.6940 0.1250
0.4940 0.1840 0.5560
0.4660 0.6740 0.1880
0.3010 0.7450 0.9330
0.6350 0.0780 0.1840
But if you draw the line using [0, 0.4470, 0.7410], you will not get the exact same color because histogram set the default transparency ( alpha ) to 0.6. So you will need to set both RGB and alpha value to get the correct color. From the documentation, you can see that plot does not have an alpha property. Therefore to get the exact same color you can use this web link to convert RGB and alpha values to a unique RGB value against a white background. The result is
[0.4000, 0.6667, 0.8431] <- This will give correct color against white background
You can check the color by plotting
plot((1:100).^2, 'Color', [0.4000, 0.6667, 0.8431], 'LineWidth', 4)
