Change default colors in paragraph block settings

Rather than adjusting the default palette, you can define (and enforce) a custom palette.

<?php
// Add a custom palette
add_action( 'after_setup_theme', 'wpse_block_color_palette' );
function wpse_block_color_palette() {
    add_theme_support(
        // Define the colors
        'editor-color-palette', array(
            // First color - black
            array(
                'name'  => esc_html__( 'Black', 'textdomain' ),
                'slug' => 'black',
                'color' => '#2a2a2a',
            ),
            // Second color - blue
            array(
                'name'  => esc_html__( 'Blue', 'textdomain' ),
                'slug' => 'blue',
                'color' => '#0000ff',
            )
            // And so on and so forth
        )
    );
}
?>

You’ll then want to add styling for the classes they’ll generate. The palette applies to any block with core styling, which boils down to “has-(slug)-background-color” and “has-(slug)-color”.

.has-black-background-color {
     background-color: #000;
 }
.has-black-color {
     color: #000;
}
.has-blue-background-color {
     background-color: #00f;
}
.has-blue-color {
     color: #00f;
}