Welcome back @russellr! I also took a long break!
The readings you are talking about are the onyomi. Meaning you’ll use those readings in words that are a compound of two kanji.
左右 - さゆう - Left and Right
左派 - さは - Left Wing
左 - ひだり - Left
ひだり and みぎ are the kunyomi for those kanji.
No さ and ゆう are not common readings for 左 and 右, and you may not ever really need to know those readings, but wanikani is trying to help best prepare you for seeing a kanji in any position you will see it. For example if you picked a different kanji, like 日, you will see both the onyomi and kunyomi a LOT so it is best to learn both. It’s just luck of the draw that みぎ is so much more common than ゆう.
Remember that usually the pink items are asking for the reading used in compound words, onyomi, the chinese reading.
And the purple if the kanji is alone is asking for the kunyomi, the japanese reading.