I have a sliver of a gap in a butt joint in my Kahrs Builders Woodloc in a gunstock color. I had asked advice from the board, which suggested the Woodwise or Timbermate. I found Woodwise locally (hard to get a good color mix) and that local dealer suggested the Bruce filler. With Kahrs being so picky about the warranty I decided to email them and ask what they recommend. Here is the answer:

"Thank you for choosing Kahrs!
You can make your own filler by mixing the following:
Get a tube of Dap Kwik Seal Tile and Tub Caulk (white, non-silicone)/
Mix with acrylic paint tints Raw Umber and Burnt Sieena (you can get these @ a craft store)
Mix the colors with the caulk to a color blending to floor color- there is no hard and fast rule here, you will have to play with it some to get the right blend/
Tip: mix on a piece of extra flooring"

What do you think?

I have done that before. It works as well as anything. It will also be more water resistant.

With the Woodwise I guess that I need to coat the filler with a polyurethane that matches the floor. Do I coat the caulk with the poly?