Source: callouts-metadata.css

Use > [!calout-name|{color}] with {color} being one of the following available color, to change the color of the callout.

Available colors

(exact colors may vary between themes)

Color	Light theme	Dark theme
white
black
gray*
brown*
red
orange
yellow
green
cyan
blue
purple
pink

You can also use the |accent option which will use the accent color. It should work with any normal callout. But it is not (yet) supported with all the custom callouts from this theme.

* not available in the standalone snippet outside of the theme.