HTML

  
  <p>Lorem, ipsum dolor sit amet consectetur <mark class="color">adipisicing
  elit.</mark> Ipsam aperiam, quod laboriosam beatae, cupiditate sint enim quam
  maxime <mark>officiis illo</mark>, harum repellendus suscipit. Tempore aut
  impedit <mark class="color">ipsam deserunt</mark> obcaecati fugit!</p>

  <p>Lorem, ipsum dolor sit amet consectetur <mark class="color">adipisicing
  elit.</mark> Ipsam aperiam, quod laboriosam beatae, cupiditate sint enim quam
  maxime officiis illo, harum repellendus suscipit. Tempore aut impedit <mark
  class="color">ipsam deserunt</mark> obcaecati fugit!>
  

CSS

  
    .color:first-of-type {
      background-color: aquamarine;
    }
    
    .color:last-of-type {
      background-color: blueviolet;
    }