In daily usage, vocabulary is the only reading that matters. It’s what you’ll see on a sign, or in a restaurant menu, or in a novel that you’re reading. A word is (usually) only pronounced one way. But the same kanji in a different word may be pronounced differently.
To illustrate with English, the words ‘cat’ and ‘civil’ are each only pronounced one way. But notice that ‘c’ is in both words, and is pronounced differently in each. ‘c’ can sound like either ‘k’ or ‘s’. That’s how it works with kanji. A kanji can have different ‘readings’ depending on what word it’s used in. But the words themselves (i.e. vocabulary) are pronounced only one way.
When Wanikani is asking for the reading of a Vocabulary word (purple background, plus the question says ‘Vocabulary’), it’s asking for how the word is really pronounced. If you enter a different reading, it’s wrong because that’s not how the word is pronounced.
When Wanikani is asking for the reading of a Kanji (pink background, plus the question says ‘Kanji’), technically any of the possible pronunciations of that kanji would be correct, but Wanikani is only asking for one specific reading, usually the most common one (which may be either the onyomi or kunyomi, so you can’t rely on that). Since any of the valid readings for that kanji are technically correct, Wanikani won’t mark it wrong if you enter the wrong one (as long as it’s one of the possible pronunciations of that kanji). Instead, it does that ‘shake’ thing and tells you that’s not the reading they’re asking for… so you can try again with the other reading.
And, of course, the light blue background is for Radicals. Radicals are only pieces of kanji, but can sometimes be the same as a whole kanji. Since their purpose is only for visually breaking down a kanji into pieces, they don’t have a reading – their purpose is purely visual. (Though sometimes several kanji that share the same radical will have the same reading).
So, to tie it back together again, let’s look at one specific radical/kanji/vocab:
力 happens to be a vocabulary word made up of only one kanji. It is pronounced ちから, which is the only way to pronounce that word.
力 as a kanji can have several pronunciations:
- In the word 力, it’s pronounced ちから.
- In the word 入力 (にゅうりょく), it’s pronounced りょく.
- In the word 馬力 (ばりき), it’s pronounced りき.
So, the three valid readings for the kanji 力 are ちから, りょく, and りき. But Wanikani specifically wants you to enter the onyomi reading (because the onyomi is more common). In this case, both りょく and りき are onyomi readings, and Wanikani will accept them. But if you accidentally enter ちから, it’s still technically correct, but it’s not the one Wanikani is asking for, so it will shake and tell you it wants the onyomi. So, there’s no harm if you enter the wrong one on kanji (as long as it’s really one of the valid readings for that kanji).
And radicals are only visual. You learn them to help you visually break down kanji into reusable chunks. 力 is used as a component of a lot of different kanji, such as 男, 助, 功, 勉… and, of course, 力 itself.