Terragen is available for Mac OS X, but not iOS, so it does not work on the iPad.
To get more color variation, try the Colour tab in the Populator, specifically the Tint Diffuse Colour function. You will need to input some kind of other node into it, for example a Power Fractal or image map, and you should also probably increase the Diffuse Colour Multiplier, perhaps a value of 2 for example.
- Oshyan