If I'm correct in interpreting "enhanced color acuity" as having 4 color receptors instead of 2 like all mammals except those few primates that have 3 (or, sometimes, another 4), then there's no reason to assume Dimetrodon didn't have that -- having 4 color receptors is the plesiomorphy for vertebrates in general, and it is assumed that mammals lost 2 as a result of a nocturnal lifestyle. About scales... at least some caecilians have scales (though I don't know whether they are more fish- or reptile-like), so the fact that most living amphibians are scaleless may be their apomorphy (evolved several times).
Good argument IMHO.
There has been the suggestion that diadectomorphs are synapsids (would offer a comfortable opportunity to separate the meanings of Synapsida and Theropsida...). The consensus is that they are the sister group to Amniota (even though they may well have laid amniotic eggs... unknown).
(Can't find the ref for the most recent phylogenetic study... Michel Laurin & Robert R. Reisz, Can. J. Earth Sci., has Solenodonsaurus janenschii in the title.)
You do know that by your suggestion you are a reptile, and Reptilia becomes the same as Amniota? :-)
The situation in amniote phylogeny is so solid that perhaps it is better to abandon the term Reptilia altogether. We have beautiful, precise terms like Amniota and Sauropsida, let's use them instead. If only because they don't carry connotations like "low vertebrate", "cold-blooded", "sluggish" etc. etc.. :-)