Block mySand = new MySand();
GameRegistry.addSubstitutionAlias("minecraft:sand", GameRegistry.Type.BLOCK, mySand);
GameRegistry.addSubstitutionAlias("minecraft:sand", GameRegistry.Type.ITEM, new ItemMultiTexture(mySand, mySand, new String[] { "default", "red", "my" }).setUnlocalizedName("sand"));
The above works for me