diff --git a/lib/configuration.js b/lib/configuration.js
index 668adaa..28b685b 100644
--- a/lib/configuration.js
+++ b/lib/configuration.js
@@ -5,6 +5,12 @@ const levels = require('./levels');
 const layouts = require('./layouts');
 const debug = require('debug')('log4js:configuration');
 
+const validColours = [
+  'white', 'grey', 'black',
+  'blue', 'cyan', 'green',
+  'magenta', 'red', 'yellow'
+];
+
 function not(thing) {
   return !thing;
 }
@@ -164,10 +170,8 @@ class Configuration {
         this.throwExceptionIf(not(anInteger(levelConfig[l].value)), `level "${l}".value must have an integer value`);
         this.throwExceptionIf(not(levelConfig[l].colour), `level "${l}" must have a 'colour' property`);
         this.throwExceptionIf(
-          not([
-            'white', 'grey', 'black', 'blue', 'cyan', 'green', 'magenta', 'red', 'yellow'
-          ].includes(levelConfig[l].colour)),
-          `level "${l}".colour must be one of white, grey, black, blue, cyan, green, magenta, red, yellow`
+          not(validColours.indexOf(levelConfig[l].colour) > -1),
+          `level "${l}".colour must be one of ${validColours.join(', ')}`
         );
       });
     }