message: docs: add instructions to colorize an external SVG
diff: -[![License](https://img.shields.io/badge/license--lightgrey.svg)](https://github.com/sumup-oss/icons/LICENSE)
  -[![Contributor Covenant](https://img.shields.io/badge/Contributor%20Covenant-v1.4%20adopted-ff69b4.svg)](CODE_OF_CONDUCT.md)
  -
  <div align="center">
  # SumUp Icons
  A collection of icons for the web, part of the SumUp Design System.
  +[![License](https://img.shields.io/badge/license--lightgrey.svg)](https://github.com/sumup-oss/icons/LICENSE)
  +[![Contributor Covenant](https://img.shields.io/badge/Contributor%20Covenant-v1.4%20adopted-ff69b4.svg)](CODE_OF_CONDUCT.md)
  +
  </div>
  ## Table of contents
  @@ -61,7 +61,7 @@ const SuccessMessage = ({ description }) => (
  );
  ```
  -To change the color of an icon, set the `color` property in CSS. The color will [cascade down](https://css-tricks.com/cascading-svg-fill-color/) since the `fill` and `stroke` attributes of most icons are set to `currentColor`. Here's an example with a CSS-in-JS library:
  +To change the color of an icon, set the `color` property in CSS. The color will [cascade down](https://css-tricks.com/cascading-svg-fill-color/) since the `fill` and `stroke` attributes of all monochrome icons are set to `currentColor`. Here's an example with a CSS-in-JS library:
  ```jsx
  import styled from '@emotion/styled';
  @@ -94,6 +94,8 @@ const SuccessMessage = ({ description }) => (
  );
  ```
  +It is not possible to change the color of an external SVG using the `css` color property. Instead, you can use the [CSS `filter` hack](https://blog.union.io/code/2017/08/10/img-svg-fill/) to colorize the icon.
  +
  ### Load from a URL
  The latest version of the icon library is [automatically deployed](https://icons.sumup-oss.now.sh/) to [Vercel Now](https://vercel.com/). The files are hosted behind a global CDN, so they load quickly for all users. You can load the icons from `https://icons.sumup-oss.now.sh/v1/<name>_<size>.svg`. Below are some examples:
  @@ -108,6 +110,8 @@ The latest version of the icon library is [automatically deployed](https://icons
  }
  ```
  +It is not possible to change the color of an external SVG using the `css` color property. Instead, you can use the [CSS `filter` hack](https://blog.union.io/code/2017/08/10/img-svg-fill/) to colorize the icon.
  +
  ## Code of Conduct
  We want to foster an inclusive and friendly community around our Open Source efforts. Like all SumUp Open Source projects, this project follows the Contributor Covenant Code of Conduct. Please, [read it and follow it](CODE_OF_CONDUCT.md).
  