How to disable automatic colors in the Twenty Twenty theme?

Building on the answer from Aristeides, here is a complete working code example of using the theme_mod_acccent_accessible_colours filter. This is tested and working in version 1.1 of TwentyTwenty and version 5.3.2 of WordPress.

You can place this code in your functions.php file and change the colours in the array to what you’d like. In the code shown below I have changed the ‘accent’ colour to a dark purple (#7f5871).

/* Hook into the colours added via the customiser. */
add_filter( 'theme_mod_accent_accessible_colors', 'op_change_default_colours', 10, 1 );

/**
 * Override the colours added in the customiser.
 *
 * @param array $default An array of the key colours being used in the theme.
 */
function op_change_default_colours( $default ) {

    $default = array(
        'content'       => array(
            'text'      => '#000000',
            'accent'    => '#7f5871',
            'secondary' => '#6d6d6d',
            'borders'   => '#dcd7ca',
        ),
        'header-footer' => array(
            'text'      => '#000000',
            'accent'    => '#7f5871',
            'secondary' => '#6d6d6d',
            'borders'   => '#dcd7ca',
        ),
    );

    return $default;
}