You can use colorbar function to have the same color on all the faces of the 3D bars as demonstrated by the following code snippet:
% Create a 3-D bar graph of data from the magic function.
Z = magic(5);
% Return the surface objects used to create the bar graph in array b.
b = bar3(Z);
% Add a colorbar to the graph.
colorbar
You an refer to this example that shows how to modify a 3-D bar plot by coloring each bar according to its height.
