diff --git a/src/main/java/GrayColor.java b/src/main/java/GrayColor.java
index e871b5d0b28213117b7f08dc11dca76de1604337..763e32618b34ca105a6c8f727688c2d18166c97a 100644
--- a/src/main/java/GrayColor.java
+++ b/src/main/java/GrayColor.java
@@ -7,4 +7,6 @@ import javafx.scene.paint.Color;
 public interface GrayColor extends Comparable<GrayColor> {
     double getLuminosity();
     Color getColor();
+    GrayColor invert();
 }
+