diff --git a/include/openmc/plot.h b/include/openmc/plot.h
index 8d9a11ff9..a71b8926c 100644
--- a/include/openmc/plot.h
+++ b/include/openmc/plot.h
@@ -35,7 +35,7 @@ extern std::vector<Plot> plots; //!< Plot instance container
 // RGBColor holds color information for plotted objects
 //===============================================================================
 
-typedef std::array<char, 3> RGBColor;
+typedef std::array<unsigned char, 3> RGBColor;
  
 typedef xt::xtensor<RGBColor, 2> ImageData;
 
diff --git a/src/plot.cpp b/src/plot.cpp
index aa649c531..e08606787 100644
--- a/src/plot.cpp
+++ b/src/plot.cpp
@@ -37,7 +37,7 @@ int n_plots;
 
 std::vector<Plot> plots;
 
-const RGBColor WHITE = {static_cast<char>(255), static_cast<char>(255), static_cast<char>(255)};
+const RGBColor WHITE {static_cast<unsigned char>(255), static_cast<unsigned char>(255), static_cast<unsigned char>(255)};
 const RGBColor NULLRGB = {0, 0, 0};
 
 //==============================================================================
@@ -740,9 +740,9 @@ void output_ppm(Plot pl, const ImageData& data)
   for (int y = 0; y < pl.pixels_[1]; y++) {
     for (int x = 0; x < pl.pixels_[0]; x++) {
       RGBColor rgb = data(x,y);
-      of.write(&rgb[RED], 1);
-      of.write(&rgb[GREEN], 1);
-      of.write(&rgb[BLUE], 1);
+      of.write(reinterpret_cast<char*>(&rgb[RED]), 1);
+      of.write(reinterpret_cast<char*>(&rgb[GREEN]), 1);
+      of.write(reinterpret_cast<char*>(&rgb[BLUE]), 1);
     }
   }
   // Close file