I recently was hanging some buff skulls and noticed that one had a quarter to half inch or so gap between the horn and skull.

The other skulls have the gap filled with some putty like stuff, dyed to match the horn.

What can I use to fill and color the gap?

And no, it isn't worth the six hour drive to my taxidermist and I have plenty of time to spend (or waste) on the project this summer.

Thanks for any suggestions!

Apoxy -- taxidermy epoxy clay. Try Mckenzie.
http://www.mckenziesp.com/

There is quite a lot of space to fill there and enough to allow many of the horns sheaths to move if they have not been filled with epoxy or fiberglass resin etc..

Make a texture mold of the horn bases by speading pure silicone on the section of the horn bases where you are wanting to "capture" the texture. Once it is dry you can peel it off and you will have a texture mold,(mould) of the horn.

Fill the horn with fiberglass resin or 2 part expandable foam first then blend in the now smaller gap with a 2 part sculpting compound like epoxy clay. You will not have to use nearly as much epoxy clay now. Add black and brown powdered tempera paints to the compound as you mix it up for a near perfect color match. Before the sculpting compound dries, press your texture mold into it to recreate the look of real horn. Blend in colors with airbrush. dust with dirt. Finished.

This is pretty much the way I do it when mounting a shoulder mount etc...
If you want the gap filled on a skull mount I would do it the same way. However, for a skull mount or Euro mount I personally would want to keep that gap there. That is the way it is. I would bond the horn sheaths to the horn cores and leave the gap around the base the way it is in nature.

Or if that seems like too much work, you can always fill with bubble gum and hit it with a can of black spray paint...

Thanks for reminding me about apoxy clay...I used to have a McKinzie catalog around here.

We'll give it a try.