The underside of my tongue is a sickly white color. assuming something related to diet rather than disease.

?: Perhaps it has always been that color. The top is velvety with sensory endings. The underside is very thin clear skin and has a white color. This in the absence of discomfort is probably normal. Those nails in the picture look a little Goth though.

It is normal: There is nothing for you to worry about. This is normal anatomical structure. This is due to differences in the vascular anatomical structure. If concerned, see your dentist for confirmation.

See doc if persists: The finger-like projections on your tongue become inflamed & collect debris, bacteria & dead cells which look white. Causes of this inflammation include dehydration, dry mouth, excessive alcohol use, fever, smoking. Patches of white can be leukoplakia (precancerous), lichen planus (autoimmune), yeast, etc. It is generally harmless but may be helped with brushing your tongue & drinking lots of H2O.