As above you can use a colored toothbrush handle or a cheap hardware store screwdriver handle that's a bright color.
The driver handles are usually made of a tough, solvent proof plastic that's better then toothbrush material
The way most people do it today is to cast an insert.
You can buy a kit, use a drop of model paint in 1 hour epoxy, or probably best, use Acrylic fingernail material from Walmart.
If the dovetail cut in the sight does not have one or two shallow holes in the bottom, use a tiny drill or a carbide Dremel cutter to cut two small holes or dimples in the sight.
These will provide locks to secure the insert so it won't come out.
Use solvent like lacquer thinner or denatured or 91% alcohol to degrease the sight.
Get two small pieces of hard plastic or metal to use as casting dams and apply a coat of wax to them to as a a release agent. Don't wipe it off.
Clamp the two pieces to the sides of the sight.
Use a tooth pick to pack some of the material into the holes in the sight.
Use a small screwdriver or a dropper to put a drop of the material into the sight cut until it's just level and let cure with the gun braced so the sight cut is level and the material won't flow out.
After it's hard remove the dams and clean up.
To use a one hour epoxy put a drop of model paint in the epoxy and mix.
Experiment to get the paint-to-mix right....too much and it'll be crumbly, to little paint and the color is pale.
To use a toothbrush or driver handle, cut a piece thin enough to fit the dovetail, and file the ends to fit the dovetail.
Use epoxy to glue into the sight.
I grind a small dimple on the bottom of the insert to make a lock to secure it.
Doing this way requires some very fine work.
If you look around you might find an insert kit of thin plastic, or a casting kit.
