Leucistic typically means 'no pigment/pattern' but the eyes are unaffected, they appear as normal (though many in reptiles are 'blue' hence you hear about 'blue eyes lucy' in ball pythons).
Hypo is a prefix meaning 'below' or 'low' thus a 'hypomelanistic' means low melanin..or low dark pigment. Thus you would expect a bearded dragon who is hypomelanistic would have little dark pigment and thus a reduction of pattern, but all other pigments would be unaffected. Dragons do have more than one pigment type, so these other 'colors' will be more prominent as they are not washed out by melanin.
I don't know if snow morphs are actually hypomelanistic or something different. bearded dragon morphs are rather complicated and not well understood for the most part. To me, a leucistic bearded dragon will be completely white (a solid milk white) with black/dark eyes. But then again I am not an expert on genetics.
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PHLdyPayne