According to Döderlein's Hand-book of Latin Synonymes (also here), canere is the more general term for music (and thus may be used for singing), whereas cantare usually is used more specifically to refer to vocal music:
Canere (from καναχεῖν) means, in the most general sense, to make
music, voce, tibiis, fidibus, like μέλπειν; cantare, with vocal music,
sententia is something you've given thought to and which relates to a given situation, a view; from this follow its uses as "thought expressed in words", "sense, sentence", "intention", "decision, judicial sentence", "moral maxim".
meā quidem sententiā ('at least in my view')
ex sententiā; praeter animī ...
There are several sources that equate albus-candidus with ater-niger, which makes niger to be the opposite of candidus - a shiny black, and ater to be the opposite of albus - a dull black. As @fdb comments it is stated explicitly in L&S dictionary (for example under candidus):
opp. niger, a glistening black; while albus is a lustreless white,
opp. ater, ...