Depending on what you mean when you say you want it to be a colored image, this might have the answer:
Of particular interest may be the use of ind2rgb() if you want the result to adhere to a given colormap.
If you actually want the output PNG to be an indexed image, that might be a different story. I can't know what values the image g contains, so I can't know how it will be mapped. For a uint8 image, things should be simple enough.
Don't use colormap() for this. Colormap() is a tool for getting/setting the current axes colormap. If you do something like colormap(jet), imread() will wind up getting some uncontrolled-length map. The length of the map will depend on the version you're using and whatever the last map was in whichever axes was most recent -- even if the image processing you're doing has nothing to do with any figures/axes.
If you want an actual Mx3 colormap, get it using the appropriate function like jet(), gray(), parula(), etc. You'll need to specify an appropriate map length.
g = imread('cameraman.tif');
% write the image with a given colormap
cmap = parula(256); % don't use colormap()
imwrite(g,cmap,'test.png')
% read it back
[g2 cmap2] = imread('test.png');
imshow(g2,cmap2)
I'm going to guess that the array g is either improperly-scaled for its class or something. I can't really know unless you show what it is though. You could always attach it as a .mat file if none of the above helps.
