Easiest way is just to modify the colors of an existing texture. I use "painted desert" found in the planes&terrains directory. One thing to remember is to try and keep the general brightness the same as the original texture. So if the color is dark brown, change it to dark gray, etc. Above are my settings in the DTE. Also if you are doing volcanic ash remember to smooth your terrain a LOT. This presents more of an appearance of snowlike stuff, which is basically what ash is (except rock fragments instead of water).