diff --git a/tests/asserthelper.cpp b/tests/asserthelper.cpp
index 1400f3b71a..8f9f36d452 100644
--- a/tests/asserthelper.cpp
+++ b/tests/asserthelper.cpp
@@ -14,6 +14,9 @@
 
 #include "asserthelper.h"
 
+namespace wxTestPrivate
+{
+
 std::ostream& operator<<(std::ostream& os, const ColourChannel& cc)
 {
     os.width(2);
@@ -22,8 +25,12 @@ std::ostream& operator<<(std::ostream& os, const ColourChannel& cc)
     return os;
 }
 
+}
+
 std::ostream& operator<<(std::ostream& os, const wxColour& c)
 {
+    using wxTestPrivate::ColourChannel;
+
     os << std::hex << std::noshowbase
        << "("
        << ColourChannel(c.Red()) << ", "
diff --git a/tests/asserthelper.h b/tests/asserthelper.h
index 74ca91242c..ce8c9c6a29 100644
--- a/tests/asserthelper.h
+++ b/tests/asserthelper.h
@@ -14,7 +14,7 @@
 #include "wx/gdicmn.h"
 #include "wx/font.h"
 
-namespace
+namespace wxTestPrivate
 {
     // by default colour components values are output incorrectly because they
     // are unsigned chars, define a small helper struct which formats them in
@@ -26,9 +26,8 @@ namespace
         unsigned char m_value;
     };
 
-} // anonymous namespace
-
-std::ostream& operator<<(std::ostream& os, const ColourChannel& cc);
+    std::ostream& operator<<(std::ostream& os, const ColourChannel& cc);
+} // wxTestPrivate namespace
 
 // this operator is needed to use CPPUNIT_ASSERT_EQUAL with wxColour objects
 std::ostream& operator<<(std::ostream& os, const wxColour& c);