You can activate Penance's ability as long as you have a card in your hand (C.R. 118.3, 602.2b, 601.2h), even if there are no black or red sources you can choose from. If there is no such source when that ability resolves, you simply don't choose any (C.R. 101.3).
Note that Penance's ability doesn't target anything (it lacks the word "target", for example) (C.R. 115.1c); just because a spell or ability has a player choose one or more objects, that doesn't necessarily make them targets of that spell or ability (C.R. 115.10a, 115.10). See also this thread, this thread and this thread.
Note that Penance's ability doesn't target anything (it lacks the word "target", for example) (C.R. 115.1c); just because a spell or ability has a player choose one or more objects, that doesn't necessarily make them targets of that spell or ability (C.R. 115.10a, 115.10). See also this thread, this thread and this thread.