Here is how i would try to do it:
% First, you need to get the image that you see as an RGB-matrix. If your
% ratioImage is an NxM matrix, you want a NxMx3 matrix where the last
% diemsion stores the three color channels red, green and blue.
rgbMat = ind2rgb(ratioImage, jet(256));
% in order to change the brightness, you need to convert these values into
% another color-space, L*a*b. The L*a*b color-space does not have 3 color
% channels (like RGB) but one channel for Lightness and two for color.
labMat = rgb2lab(rgbMat);
% Now you can change the values in the first channel of labMat to adjust
% the brightness.
labMat(:, :, 1) =
% after you are done, convert back to RGB and display
rgbMat = lab2rgb(labMat);
imshow(rgbMat)
On a different note:
Many people object to the use of the colormap jet. It produces very colorful images, but at the cost accurate interpretation of your data, because the contrast is not uniform. Hence, a couple of years ago matlab introduced a new (default) colormap 'parula'. Steve Eddins wrote about their rationale in these blog posts:
But since you additionally change the brighntess depending on values in another matrix, these considerations are maybe not necessary your case.
