What is the real purpose of the `value` attribute of radio buttons?

(Yes, I have read the related articles on w3schools and MDN, but it is still unclear.)

In other words, which set of values should be assigned to the `value` attributes in the example below:

* `Red`, `Yellow`, `Green`, `Red`, `Yellow`, `Green` **or**
* `color1`, `color2`, `color3`, `color4`, `color5`, `color6` **or**
* `red1`, `yellow1`, `green1`, `red2`, `yellow2`, `green2`**?**

```html
<article>
  <h1>Saturday</h1>

  <p>Which color you prefer on Saturdays?</p>

  <input type="radio" name="saturday-color" id="color1" value="?">
  <label for="color1">Red</label>

  <input type="radio" name="saturday-color" id="color2" value="?">
  <label for="color2">Yellow</label>

  <input type="radio" name="saturday-color" id="color3" value="?">
  <label for="color3">Green</label>
</article>

<article>
  <h1>Sunday</h1>

  <p>Which color you prefer on Sundays?</p>

  <input type="radio" name="sunday-color" id="color4" value="?">
  <label for="color4">Red</label>

  <input type="radio" name="sunday-color" id="color5" value="?">
  <label for="color5">Yellow</label>

  <input type="radio" name="sunday-color" id="color6" value="?">
  <label for="color6">Green</label>
</article>
```