By default, the color limits of an axis are set to span the maximum range of plotted objects that could use a colormap. In your case, contour plots can use the colormap to color lines, and even though you changed this behavior by setting the contour line color to blue, the colormap is still adjusting itself to the limits of that data.
You can fix it by simply changing the axis color limits:
set(gca, 'clim', [min(Ci(:)) max(Ci(:))]);
Now, if you actually want to display a separate colormap for the contour lines than for the underlying geoshow plot, then you'll have to use something like freezeColors (older Matlab) or to layer axes on top of each other with different colormaps (R2014b+). But that isn't necessary in your example.