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Edit your theme’s style.css.

And add the following:
#wp-admin-bar .padder, #wp-admin-bar ul li ul {background:red !important;}

This is just to get you started.

Again, I advise you to create a child theme to apply your new style changes. Read this guide to build a child theme from the default BP theme:
https://codex.buddypress.org/how-to-guides/building-a-buddypress-child-theme/