You can use two linked axes with different colormaps. Example:
% Axes
ax(1)=axes('color','none','xcolor','none','ycolor','none');hold on
ax(2)=axes('color','none','xcolor','none','ycolor','none','ydir','reverse');hold on
linkaxes(ax,'xy')
axis([-3 3 -3 3])
% Contour data
[X,Y,Z] = peaks;
% Image
axes(ax(2))
I = imread('cameraman.tif');
imagesc(I,'XData',[min(X(:)) max(X(:))],'YData',[min(Y(:)) max(Y(:))]);
colormap(ax(2),'gray')
% Contour
axes(ax(1))
h = contour(X,Y,peaks);hold on
colormap(ax(1),'parula')
Just make sure the axes have the same position. If you create a colorbar "outside" of the axes area, then one axis will shrink. Usually I just create the colorbar "inside" and then move it outside by adapting its position properties.
