The output to scatterhist() is a vector of handles to the scatter plot, the x histogram, and the y histogram axes, respectively. To change the face color of a bar plot, you just need to get the children of the histogram axis and then change the FaceColor. The example below changes the face color of the x axis histogram.
h = scatterhist(...);
h(2).Children(1).FaceColor = 'r'
