The basic rule is that players from Shirtless Team are always shirtless, but players from Shirt Team don't always wear shirts. Sometimes, at the end of the day, it gets too hot and they take off their shirts. They are still players from Shirt Team, but right now they’re shirtless, and look just like players of the other team.
Consequently, if you see a player wearing a shirt, you know they’re from Shirt Team; but if you see a shirtless player, you can’t be sure. They may be from either team. You have to rely on some other information (e.g. which side of the field are they playing on).
Example: The word Tage, “days“, is made from the phonemes /ˈtɑːɡə/. The third one is a voiced obstruent, /ɡ/. You’ve seen that voiced obstruents can be (=sometimes are) voiceless; in German this happens, for example, in word-final position. So take away the plural -e, making Tag “day”, and now it’s pronounced [tɑːk]. Left alone at the end of the word, the /ɡ/ “takes off its shirt”, so to speak. But it’s still a Team Voiced player. How can you tell? Well just add a vowel and you’ll see its voiced form.
Compare the word Volk /fɔlk/ “folk; people”. Its plural form is Völker, /ˈfœlkɐ/. Between /fɔlk/ and /ˈfœlkɐ/, the /k/ never changes shape. That’s why it makes sense to say that Voiced consonants can be voiceless (sometimes), but Voiceless consonants are never voiced.
(What I am trying to convey with capital letters and with the "team" metaphor is the difference between phonemes and phonetic realization, as pointed by Jeremy Needle above.)