Difference between 'Candor' and 'Sincerity'

In English, the word candor [candour in many dialects] has come to primarily mean openness and frankness, and a tendency to tell harsh truths. The OED defines it as:

Freedom from reserve in one's statements; openness, frankness, ingenuousness, outspokenness.

Sincerity does mean something similar, but it doesn't generally have the "tendency to tell harsh truths" connotation.

Freedom from dissimulation or duplicity; honesty, straightforwardness.

In other words, I would use candor to describe someone who speaks up about harsh truths without necessarily being asked. Sincerity would describe someone who can be relied upon to tell the truth when asked, though they might not speak up on their own. (This is why candor is not always viewed positively, since it can cause social friction; whereas sincerity is generally seen as an unalloyed virtue.)

Aside: To my surprise, the use of candor to mean

Stainlessness of character; purity, integrity, or innocence.

(which would seem to be the fashion it is used in Portuguese) can be found in older English texts, though the OED now considers this usage "obsolete".

'Sincerity' is being truthful in what is said; candour is the willingness to say what is truthful.