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I believe there is also track available with gray 'concrete' ties as well. The black ones are typically cheaper, likely because they're cheaper to produce.
Artieiii pointed out he has Atlas brown tie tracks. I've seen them available online and in stores. Once upon a time I read a comparison of different brands' track measurements. They listed the differences between, for example:
Atlas code 100 black ties
Atlas code 83 black ties
Atlas code 83 brown ties
Peco code 100 gray ties
Peco code 100 brown ties
Peco code 83 brown ties
I believe they had millimeter measurements for all dimensions of 20+ different flextrack products. It was pretty cool.
Fresh tie are coated with creosote, which turns the ties a blackish-brown color. As the ties weather, they tend to turn a grayish color. I've never seen really brown ties on the real tracks.
A pile of new ties:
A section of 1:1 scale "Snap Track":
1:1 scale Snap Switch:
Steel ties:
Most folks will paint their track anyway... rusty rails, and whatever looks good to them for the ties. After ballasting, you won't see much of the ties anyway. Use what looks good to you, black, gray or brown.