How can I invert color using CSS?

Add the same color of the background to the paragraph and then invert with CSS:

div {
    background-color: #f00;
}

p { 
    color: #f00;
    -webkit-filter: invert(100%);
    filter: invert(100%);
}

<div>
    <p>inverted color</p>
</div>

Here is a different approach using mix-blend-mode: difference, that will actually invert whatever the background is, not just a single colour:

div {
  background-image: linear-gradient(to right, red, yellow, green, cyan, blue, violet);
}
p {
  color: white;
  mix-blend-mode: difference;
}

<div>
  <p>Lorem ipsum dolor sit amet, consectetur adipiscit elit, sed do</p>
</div>

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