This is wrong:
Why? See Steve's blog:
It should be
[rows, columns, numberOfSlices] = size(Image);
but don't use Image as the name of your variable since that's a built-in function (almost). Also, the pet/spect image is already grayscale. You shouldn't need to convert to gray scale.
Also, you did not make a color image, you made a volumetric image with 60 gray scale slices in it.
I'm not sure what you're trying to do, but what you did doesn't make sense to me.
If you're trying to make a volumetric image with 60 RGB slices in it instead of 60 gray scale images, you'll need a 4-D image where the 4th dimension is the slice number and the third dimension is the color channel (1, 2, or 3).