This is a tool for practicing and understanding value. Generate a random color and try to match up it's value with the slider. After you've chosen a grey you can show the answer by clicking the eye icon in the middle. The middle box will turn into the correct greyscale value so you can compare your guess to the answer.
Value is the measure of how light or dark a given hue can be. It is one of the foundational building blocks of color theory alongside hue and saturation. Beyond color theory, developing an eye for value will always benefit your compositional skills. We use value to lay out our paintings, compose our photographs, and even choose our living room sofa. Being able to see what value every color is will make it a breeze to choose colors that strengthen your work, whatever that work may be. And while it's easy to notice that a fully saturated yellow has a lighter value than a fully saturated blue, there are some pretty difficult grey areas (no pun intended) that even professional artists struggle with. So in short: Understanding value will strengthen your understanding of color and lead to better compositions!
Due to rounding shenanigans on some of values or answer percentages, the answers are sometimes just a tad off. As far as I could see the inaccuracy is never larger than 1%, which is acceptable for learning to recognise values. So while you can take the answer with a grain of salt every once in a while, it's a very small grain of salt.