diff --git a/src/DGtal/io/colormaps/GradientColorMap.ih b/src/DGtal/io/colormaps/GradientColorMap.ih
index 7fe79b1786..b703bb91e1 100644
--- a/src/DGtal/io/colormaps/GradientColorMap.ih
+++ b/src/DGtal/io/colormaps/GradientColorMap.ih
@@ -225,7 +225,7 @@ DGtal::GradientColorMap<PValue,PDefaultPreset,PDefaultFirstColor,PDefaultLastCol
   out << "[GradientColorMap "
       << " min=" << myMin
       << " max=" << myMax;
-  std::vector<Color>::iterator it = myColors.begin();
+  std::vector<Color>::const_iterator it = myColors.begin();
   while ( it != myColors.end() ) {
     out << " RGB("
   << it->red() << ","