Accessing pseudo elements via JavaScript or jQuery is often painful/impossible. However, accessing their styles is fairly simple.

Using getComputedStyle

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First, find the element in question.

let element = document.querySelector('.my-element') // or $('.my-element').get(0) when using jQuery

Next, use JavaScript's getComputedStyle. It takes an optional 2nd argument to filter for pseudo elements.

let style = window.getComputedStyle(element, '::before')
let color = style.getPropertyValue('background-color')

The returned color will be a string with a color value like "rgb(253, 126, 20)".

Bonus: Hex color strings

If you want to convert that to a hex color, here are two solutions.

JavaScript

Modern JavaScript (ES2017+) can do it like this:

let hexColor = rgbColor
  .match(/(\d+), (\d+), (\d+)/)
  .slice(1, 4)
  .map((number) => { return Number(number).toString(16).padStart(2, '0') })
  .join('')
  .replace(/^/, '#')

Ruby

My use case was comparing colors in an integration test, so I used JS to get the pseudo element's style via Selenium, and then processed it in my Ruby code like so:

hex_color = rgb_color
  .match(/(\d+), (\d+), (\d+)/)
  .captures
  .map { |number| number.to_i.to_s(16).rjust(2, '0') }
  .join
  .prepend('#')