How to change the font-colour of categories?

Hello Julia !
If you want to change the color of the content in the categories, you can do as the following:
1. Install the Jetpack by WordPress.com plugin.
2. Set up Jetpack then enable the Custom CSS feature.
3. Move through Appearance > Editor Css, add the following code:

   .category .entry-content {
    color: #000000;
}

If you still face the issue, please send me a screenshot for further checking.
Hope this helps !