All Colt Accro and Ellison sights are interchangeable on all Colt pistols with the standard Colt sight cut.
So, yes, an Ellison sight will fit a Diamondback.
Here's the story on Diamondback and Python front sights.
The basic sight design, shape, and height is the same for both models.
The sight on the 2 1/2" Python is 1/10" thick, all others are 1/8".
The 4" Diamondback sight is machined as part of the barrel, and is NOT removable.
The 6" Diamondback sight is pinned in with a single pin.
The Python 4" and 6" front sights will interchange with the 6" Diamondback.
The only difference is, the Diamondback sight has a single mounting pin hole in the front, and the Python has two holes.
Here's the factory part numbers for the sights:
Diamondback 6". #56624
Python 4" and 6". #51868
Python red ramp. #55405B
Python Patridge. #53960B
You can fit a Diamondback sight to a Python by drilling the extra hole in the rear.
You can fit a Python Sight to a Diamondback by just inserting the front pin.
NOTE: The NEW MODEL Python sights have a single pin sight, which I assume is just the standard single hole Diamondback sight blade.
As far as I know, when Colt went to the new style threaded barrels, they also went to the single pin Diamondback sight.
This was cheaper, AND ID'd the new thread barrels.
Bottom line, the Ellison sight will "drop-in" the Diamondback, BUT you'll probably need the higher Patridge front to avoid having the rear cranked up a little too high.
Some guns work fine with the standard blade sight, some require raising the rear a little higher than you might want.
You can use either a Diamondback or old style Python sight on the Diamondback, by just inserting the single front pin.
New style Python sights appear to be just Diamondback sights.