diff --git a/openmc/universe.py b/openmc/universe.py
index 5b8c2bc8aa..f3f95aebbe 100644
--- a/openmc/universe.py
+++ b/openmc/universe.py
@@ -337,7 +337,8 @@ class Universe(UniverseBase):
             plot.pixels = pixels
             plot.basis = basis
             plot.color_by = color_by
-            plot.colors = colors
+            if colors is not None:
+                plot.colors = colors
             model.plots.append(plot)
 
             # Run OpenMC in geometry plotting mode