Styling:

- support references in color functions
- added test for using color functions in styling

flatlaf-core/src/main/java/com/formdev/flatlaf/FlatLaf.java

@@ -32,6 +32,7 @@ import java.beans.PropertyChangeListener;
 import java.io.File;
 import java.lang.reflect.Method;
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.Enumeration;
 import java.util.HashMap;
 import java.util.List;
@@ -796,7 +797,8 @@ public abstract class FlatLaf
 		throws IllegalArgumentException
 	{
 		// parse value
-		Object val = UIDefaultsLoader.parseValue( key, value, valueType );
+		Object val = UIDefaultsLoader.parseValue( key, value, valueType, null,
+			v -> UIDefaultsLoader.resolveValueFromUIManager( v ), Collections.emptyList() );
 
 		// create actual value if lazy or active
 		if( val instanceof LazyValue )

flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java

@@ -290,6 +290,25 @@ class UIDefaultsLoader
 		return resolveValue( newValue, propertiesGetter );
 	}
 
+	static String resolveValueFromUIManager( String value ) {
+		if( !value.startsWith( PROPERTY_PREFIX ) )
+			return value;
+
+		String key = value.substring( PROPERTY_PREFIX.length() );
+		Object newValue = UIManager.get( key );
+		if( newValue == null )
+			throw new IllegalArgumentException( "property '" + key + "' not found" );
+
+		// convert binary color to string
+		if( newValue instanceof Color ) {
+			Color color = (Color) newValue;
+			int alpha = color.getAlpha();
+			return String.format( (alpha != 255) ? "#%06x%02x" : "#%06x", color.getRGB() & 0xffffff, alpha );
+		}
+
+		throw new IllegalArgumentException( "property value type '" + newValue.getClass().getName() + "' not supported in references" );
+	}
+
 	enum ValueType { UNKNOWN, STRING, BOOLEAN, CHARACTER, INTEGER, FLOAT, BORDER, ICON, INSETS, DIMENSION, COLOR,
 		SCALEDINTEGER, SCALEDFLOAT, SCALEDINSETS, SCALEDDIMENSION, INSTANCE, CLASS, GRAYFILTER, NULL, LAZY }
 

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatStylingSupport.java

@@ -191,6 +191,7 @@ public class FlatStylingSupport
 	}
 
 	private static Object parseValue( String key, String value ) {
+		// simple reference
 		if( value.startsWith( "$" ) )
 			return UIManager.get( value.substring( 1 ) );
 
@@ -199,6 +200,7 @@ public class FlatStylingSupport
 		if( key.startsWith( "[" ) )
 			key = key.substring( key.indexOf( ']' ) + 1 );
 
+		// parse string
 		return FlatLaf.parseDefaultsValue( key, value, null );
 	}