I would recommend using the built-in tools in FT. On the rainfall tab of Lighting and Color, set # colors to 256. Then click the Lowest box and select black. Then, while holding down the Shift key, click on the Highest Value and (still holding down the Shift key) click white. The color list should be a full grayscale value.
This technique works for any two colors, because Shift triggers use of full RGB colors rather than the more limited Campaign Cartographer palette when picking and interpolating colors. Doing the above steps without holding down Shift will get you a 16-color step; holding down Shift gets the full 256-color ramp.