no, हाथों as a plural is only used in the oblique (with postpositions like का or को etc.). The regular form of the plural is just हाथ. So it would be: उसके हाथ लाल हैं
No. Colour is the subject of the sentence. You would say "are" if you were just saying "His hands are red." If you take out "of his hands" then it would be "The colour is red."