No, you really can't have the masthead Tricolor/anchor as your only navigation lights.
The COLREGS require that you have a steaming light for when you are under power, and that requires a white light a meter or more above the sidelights--so a tricolor/anchor light wouldn't meet the requirements under power.
Also, from a safety standpoint, deck level lights are far safer and easier to see in a crowded harbor traffic situation. Most small powerboats don't look up high enough to see mast-top mounted navigation lights, and if those are your only lights, you'll be at serious risk of getting hit.
You can get special fasteners that would make removing the deck level fixtures much more difficult. They sell stainless steel "security" screws for just such a purpose. However, you'll probably want to keep the specialized tools you'll need to remove them aboard the boat, so you can work on the lights as necessary. Ideally, you'll want four light fixtures to handly your navigation lighting.
1) Mast top mounted tricolor/anchor light. The new LED-based ones only require two wires for both functions, so this is relatively easy to do and generally doesn't require running more wire, even if you only had an anchor light up there in the first place.
2) Masthead or steaming light, usually about the height of the first spreaders. This is often combined with a foredeck light, for working on the foredeck at night.
3) Bow bicolor deck-level navigation lights
4) White stern light.
In harbor situations, you would use the deck level lights. If motoring in a crowded harbor, you'd also turn on the masthead light.
Out at sea, you'd use the tricolor light if under sail, or the bicolor and the anchor light (provided your boat is small enough) when under power. No stern light would be necessary for boats under 12 meters IIRC, since they're allowed to us an all-around 360 white light for both the steaming light and the stern light.
At anchor, you'd use the anchor light, but might also want to rig a light lower down, to better illuminate your boat in close traffic situations.
This maximizes your visibility in all situations. This also gives you some redundancy, in the case of a nav light fixture being damaged or the LED/bulb failing.
I want to replace my6 navigation lights with some LED nav lights. I have read where people have bought a tri-color steaming light(I think thats right) and it mounts at the top of the mast.
Since it has both red and green lights and a white bulb, I was curious if it is legal to only have that one light for all of your nightime needs underway/at anchor.
I think this would be the best way to go LED if you can use that one light for everything. So can I?