Putting new gravel on top of the old one is a great way to retain the beneficial bacteria. However, if there is much color difference between the two gravel types, then you'll still have a tank bottom looking unnaturally. Sooner or later you'll have to vacuum your gravel and you can't do that effectively without stirring the gravel. Once you stir the gravel, it'll be possible to see the unnaturally looking gravel. Unless you don't care about that, another thing you can do is remove the old gravel, put the new one in, AND take some of the old gravel and scatter it across the old gravel - this way the beneficial bacteria from the old gravel will faster colonize the new gravel. If you don't want to mix the 2 gravels, put the old gravel in some aquarium safe container, and then put that container on top of the new gravel. When the bacteria have colonized the new gravel, all you'll have to do will be to remove the container
