You can choose the color from any sheath in the gallery, it'll give you a much better idea of what the whole sheath will look like. But below is also a flat swatch color chart you can pick the color from.

The first row are the recommended colors, because it is the best kind of dye.

Reasonable effort was made to make the swatches look correctly on a PC monitor, but as a reference at the bottom of the chart there are the following samples: