diff --git a/src/methods.cc b/src/methods.cc
index 9e1fe1c..821928f 100644
--- a/src/methods.cc
+++ b/src/methods.cc
@@ -257,9 +257,8 @@ void ImageManipulator::threaded_get_color(int const t_h)
            reference_.at<uchar>(t_h, w + 2)};
     auto pos = std::find(std::begin(colors_), std::end(colors_), temp);
     if (pos == std::end(colors_)) {
-      colors_mutex_.lock();
+      std::lock_guard<std::mutex> lock(colors_mutex_);
       colors_.push_back(std::move(temp));
-      colors_mutex_.unlock();
     }
   }
 }