The whole color scale that you specify, whether jet or hot or parula or autumn or whatever, will be applied between the min you pass to caxis and the max you pass to caxis. Values of the array higher than the max will appear as the top color, and values less than the min will appear with the bottom color.
Try this demo and maybe it will help you understand:
hFig = figure(); % Bring up a new figure (unnecessary if none exist yet).
z = peaks(30);
zMin = min(z(:))
zMax = max(z(:))
subplot(1,2,1);
surf(z);
colormap(jet(256));
colorbar;
% Set up figure properties:
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
% Get rid of tool bar and pulldown menus that are along top of figure.
set(gcf, 'Toolbar', 'none', 'Menu', 'none');
% Give a name to the title bar.
set(gcf, 'Name', 'Demo by ImageAnalyst', 'NumberTitle', 'Off')
subplot(1,2,2);
imshow(flipud(z), []);
axis on;
set(gca, 'ydir', 'reverse');
colormap(jet(256));
colorbar;
caxis([zMin, zMax]);
uiwait(msgbox('Now we will make the whole color scale go from -7 to 0 so it will show only negative values in the image on the right'));
subplot(1,2,1);
caxis([-7, 0]);
subplot(1,2,2);
caxis([-7, 0]);
