diff --git a/util/Color.cc b/util/Color.cc
index b58a6fe..fc99dc3 100644
--- a/util/Color.cc
+++ b/util/Color.cc
@@ -43,6 +43,7 @@ Color & Color::operator+=(const Color & other)
     r += other.r;
     g += other.g;
     b += other.b;
+    return *this;
 }
 
 Color & Color::operator-=(const Color & other)
@@ -50,6 +51,7 @@ Color & Color::operator-=(const Color & other)
     r += other.r;
     g += other.g;
     b += other.b;
+    return *this;
 }
 
 Color operator+(const Color & c1, const Color & c2)