I know that Word's HTML capabilities are limited, but I'm not sure if it excludes colour or not.
At any rate, I know for a fact that even up to 2013, Citations and Bibliographies are not aware of each other (i.e., when calling for the template for a bibliography, Word does not tell the template about sources which have already been cited).
Unfortunately, that pretty much puts a spanner in the works of colouring only things which have been cited, but you may be able to add colour at any rate.
At any rate, I know for a fact that even up to 2013, Citations and Bibliographies are not aware of each other (i.e., when calling for the template for a bibliography, Word does not tell the template about sources which have already been cited).
Unfortunately, that pretty much puts a spanner in the works of colouring only things which have been cited, but you may be able to add colour at any rate.