diff --git a/cpp/open3d/visualization/gui/Util.h b/cpp/open3d/visualization/gui/Util.h
index 0bc34d7972c..fee82ff0cfb 100644
--- a/cpp/open3d/visualization/gui/Util.h
+++ b/cpp/open3d/visualization/gui/Util.h
@@ -37,6 +37,10 @@ namespace gui {
 
 class Color;
 
+// This one function is here, because ImVec4 requires imgui.h, and can't be
+// forward-declared because we need to know the size, since it is a return
+// value. Since imgui.h is an implementation detail, we can't put this function
+// in Color or it would pull in imgui.h pretty much everywhere that gui is used.
 ImVec4 colorToImgui(const Color& color);
 
 }  // namespace gui