diff --git a/src/wp-includes/class-wp-theme-json.php b/src/wp-includes/class-wp-theme-json.php
index 555898370f863..d59206f3fecf5 100644
--- a/src/wp-includes/class-wp-theme-json.php
+++ b/src/wp-includes/class-wp-theme-json.php
@@ -1925,6 +1925,48 @@ public function get_styles_block_nodes() {
 		return static::get_block_nodes( $this->theme_json );
 	}
 
+	/**
+	 * Returns a filtered declarations array if there is a separator block with only a background
+	 * style defined in theme.json by adding a color attribute to reflect the changes in the front.
+	 *
+	 * @since 6.1.1
+	 *
+	 * @param array $declarations List of declarations.
+	 * @return array $declarations List of declarations filtered.
+	 */
+	private static function update_separator_declarations( $declarations ) {
+		$background_matches = array_values(
+			array_filter(
+				$declarations,
+				static function( $declaration ) {
+					return 'background-color' === $declaration['name'];
+				}
+			)
+		);
+
+		if ( isset( $background_matches[0]['value'] ) ) {
+			$border_color_matches = false;
+			$text_color_matches   = false;
+
+			foreach ( $declarations as $declaration ) {
+				if ( 'border-color' === $declaration['name'] ) {
+					$border_color_matches = true;
+				} elseif ( 'color' === $declaration['name'] ) {
+					$text_color_matches = true;
+				}
+			}
+
+			if ( ! $border_color_matches && ! $text_color_matches ) {
+				$declarations[] = array(
+					'name'  => 'color',
+					'value' => $background_matches[0]['value'],
+				);
+			}
+		}
+
+		return $declarations;
+	}
+
 	/**
 	 * An internal method to get the block nodes from a theme.json file.
 	 *
@@ -2101,6 +2143,11 @@ function( $pseudo_selector ) use ( $selector ) {
 			}
 		}
 
+		// Update declarations if there are separators with only background color defined.
+		if ( '.wp-block-separator' === $selector ) {
+			$declarations = static::update_separator_declarations( $declarations );
+		}
+
 		// 2. Generate and append the rules that use the general selector.
 		$block_rules .= static::to_ruleset( $selector, $declarations );
 
diff --git a/tests/phpunit/tests/theme/wpThemeJson.php b/tests/phpunit/tests/theme/wpThemeJson.php
index 56b1bec0e138c..5f373b52e2053 100644
--- a/tests/phpunit/tests/theme/wpThemeJson.php
+++ b/tests/phpunit/tests/theme/wpThemeJson.php
@@ -3997,4 +3997,96 @@ function data_set_spacing_sizes_when_invalid() {
 			),
 		);
 	}
+
+	/**
+	 * Tests the core separator block outbut based on various provided settings.
+	 *
+	 * @ticket 56903
+	 *
+	 * @dataProvider data_update_separator_declarations
+	 *
+	 * @param array $separator_block_settings Example separator block settings from the data provider.
+	 * @param array $expected_output          Expected output from data provider.
+	 */
+	public function test_update_separator_declarations( $separator_block_settings, $expected_output ) {
+		// If only background is defined, test that includes border-color to the style so it is applied on the front end.
+		$theme_json = new WP_Theme_JSON(
+			array(
+				'version' => WP_Theme_JSON::LATEST_SCHEMA,
+				'styles'  => array(
+					'blocks' => array(
+						'core/separator' => $separator_block_settings,
+					),
+				),
+			),
+			'default'
+		);
+
+		$stylesheet = $theme_json->get_stylesheet( array( 'styles' ) );
+
+		$this->assertSame( $expected_output, $stylesheet );
+	}
+
+	/**
+	 * Data provider for separator declaration tests.
+	 *
+	 * @return array
+	 */
+	function data_update_separator_declarations() {
+		return array(
+			// If only background is defined, test that includes border-color to the style so it is applied on the front end.
+			'only background'                      => array(
+				array(
+					'color' => array(
+						'background' => 'blue',
+					),
+				),
+				'expected_output' => 'body { margin: 0; }.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }.wp-site-blocks > .alignright { float: right; margin-left: 2em; }.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }.wp-block-separator{background-color: blue;color: blue;}',
+			),
+			// If background and text are defined, do not include border-color, as text color is enough.
+			'background and text, no border-color' => array(
+				array(
+					'color' => array(
+						'background' => 'blue',
+						'text'       => 'red',
+					),
+				),
+				'expected_output' => 'body { margin: 0; }.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }.wp-site-blocks > .alignright { float: right; margin-left: 2em; }.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }.wp-block-separator{background-color: blue;color: red;}',
+			),
+			// If only text is defined, do not include border-color, as by itself is enough.
+			'only text'                            => array(
+				array(
+					'color' => array(
+						'text' => 'red',
+					),
+				),
+				'expected_output' => 'body { margin: 0; }.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }.wp-site-blocks > .alignright { float: right; margin-left: 2em; }.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }.wp-block-separator{color: red;}',
+			),
+			// If background, text, and border-color are defined, include everything, CSS specifity will decide which to apply.
+			'background, text, and border-color'   => array(
+				array(
+					'color'  => array(
+						'background' => 'blue',
+						'text'       => 'red',
+					),
+					'border' => array(
+						'color' => 'pink',
+					),
+				),
+				'expected_output' => 'body { margin: 0; }.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }.wp-site-blocks > .alignright { float: right; margin-left: 2em; }.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }.wp-block-separator{background-color: blue;border-color: pink;color: red;}',
+			),
+			// If background and border color are defined, include everything, CSS specifity will decide which to apply.
+			'background, text, and border-color'   => array(
+				array(
+					'color'  => array(
+						'background' => 'blue',
+					),
+					'border' => array(
+						'color' => 'pink',
+					),
+				),
+				'expected_output' => 'body { margin: 0; }.wp-site-blocks > .alignleft { float: left; margin-right: 2em; }.wp-site-blocks > .alignright { float: right; margin-left: 2em; }.wp-site-blocks > .aligncenter { justify-content: center; margin-left: auto; margin-right: auto; }.wp-block-separator{background-color: blue;border-color: pink;}',
+			),
+		);
+	}
 }