Index: /trunk/src/org/openstreetmap/josm/data/Preferences.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/Preferences.java	(revision 11057)
+++ /trunk/src/org/openstreetmap/josm/data/Preferences.java	(revision 11058)
@@ -18,4 +18,5 @@
 import java.lang.reflect.Field;
 import java.nio.charset.StandardCharsets;
+import java.util.AbstractMap;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -38,4 +39,5 @@
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
+import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
@@ -1526,4 +1528,8 @@
                 "validator.org.openstreetmap.josm.data.validation.tests.MapCSSTagChecker.entries",
                 "resource://data/validator/power.mapcss");
+        // drop in March 2017
+        if (loadedVersion < 11058) {
+            migrateOldColorKeys();
+        }
 
         for (String key : OBSOLETE_PREF_KEYS) {
@@ -1533,4 +1539,23 @@
             }
         }
+    }
+
+    private void migrateOldColorKeys() {
+        settingsMap.keySet().stream()
+                .filter(key -> key.startsWith("color."))
+                .flatMap(key -> {
+                    final String newKey = ColorProperty.getColorKey(key.substring("color.".length()));
+                    return key.equals(newKey) || settingsMap.containsKey(newKey)
+                            ? Stream.empty()
+                            : Stream.of(new AbstractMap.SimpleImmutableEntry<>(key, newKey));
+                })
+                .collect(Collectors.toList()) // to avoid ConcurrentModificationException
+                .forEach(entry -> {
+                    final String oldKey = entry.getKey();
+                    final String newKey = entry.getValue();
+                    Main.info("Migrating old color key {0} => {1}", oldKey, newKey);
+                    put(newKey, get(oldKey));
+                    put(oldKey, null);
+                });
     }