Proposed solution for issue #1736. Adding public chart api as context of the color callback

c3.js

@@ -5801,7 +5801,7 @@
                 color = pattern[ids.indexOf(id) % pattern.length];
                 colors[id] = color;
             }
-            return callback instanceof Function ? callback(color, d) : color;
+            return callback instanceof Function ? callback.call($$.api,color, d) : color;
         };
     };
     c3_chart_internal_fn.generateLevelColor = function () {

src/color.js

@@ -22,7 +22,7 @@ c3_chart_internal_fn.generateColor = function () {
             color = pattern[ids.indexOf(id) % pattern.length];
             colors[id] = color;
         }
-        return callback instanceof Function ? callback(color, d) : color;
+        return callback instanceof Function ? callback.call($$.api,color, d) : color;
     };
 };
 c3_chart_internal_fn.generateLevelColor = function () {