Changed the MDPalette definition to use a union.

angular-material/angular-material.d.ts

@@ -150,8 +150,8 @@ declare module angular.material {
         A400?: string;
         A700?: string;
         contrastDefaultColor?: string;
-        contrastDarkColors?: string[];
-        contrastLightColors?: string[];
+        contrastDarkColors?: string|string[];
+        contrastLightColors?: string|string[];
     }
 
     interface MDThemeHues {