What would be the result if a color-sound synesthete were to go color blind due to diabetes? Color blindness may be caused by diabetes. Would a color-sound synesthete lose her/his ability to hear two different tones when he/she is watching, let's say, magnets:
?
To whatever extent colorblindness is a consequence of sensation loss, perceptual loss should necessarily follow. If colorblindness results from diabetes solely due to retinal damage, this means diabetes prevents these two colors from causing different sensory stimulation. If the sensory stimulation is truly the same, it should be perceived the same. A signal (red) that cannot be discriminated from another signal (green) in terms of the basic sensation information received at the retina (stimulation of the same sets of cones, for lack of a more specially sensitive alternative) cannot be better discriminated in perceptual processes that occur thereafter in the brain, unless there's some other source of discriminating information involved in that perceptual process (I doubt there is in this case).

To answer @ChuckSherrington's expansion of the question, common cases of genetic colorblindness involve abnormal function at the retina as well, so there is likely to be some similarity. I'm unaware of any inherited conditions resulting in colorblindness through abnormal perceptual function (in the brain). However, the Wikipedia page draks linked lists quite a variety of manifestations for genetic colorblindness, so there probably are some subtle differences between the consequences of some genetic abnormalities and the consequences of diabetes. For instance, it seems genetically-based loss of color vision can affect one or more colors, partially or completely. Maybe diabetes-based symptoms are similarly various at certain stages! It is a degenerative process that can result in complete blindness, but which aspects of visual sensation are the first to go may vary across individual cases.