Changing Category Block Colours on Home Page

Try adding this CSS:

/* change post thumbnail category links to blue */
.tp-post-thumbnail .cat-links ul li a {
    background: #00f;
}

To add custom CSS:

• If you have a child theme you can add the CSS to the style.css file.
• If you are using WordPress 4.7+ you can use the Additional CSS option in the Customizer.
• If your theme has a custom CSS option you can use that to add the CSS.
• Install a plugin like https://www.ads-software.com/plugins/simple-css/.

Thank you very much, that solved the problem perfectly. ??