The best way to achieve your result is to think about what is happening in your reference photo - what time of day it was taken at, what angle (toward or away from the sun), how high the clouds might be, etc.
In your specific case I would guess it is either dusk or dawn, before the sun has risen or after it has set. And there is actually very little color on the clouds. So the first thing I tried is just setting the sun to -5 Elevation. That makes it quite dark, but in fact your photo reference probably shows an image with a longer exposure to increase the brightness, so I adjust the exposure in Terragen to compensate. That got me pretty close to the basic look.
Then I adjusted haze amount (increased to 2), Bluesky Horizon Colour (slightly more purple and a little more saturation), and then Redsky Decar Colour (just trying to match what is seen in the reference photo). I didn't nail it, but this is just a quick attempt to get a similar look.
Finally I added some clouds to roughly mimic the basic composition and look in the reference image, but certainly the clouds could be brought a lot closer to the photo with some more tweaking.
Attached are the resulting image and a TGD.
Again the lesson here is that Terragen is generally set up to give you output that matches the real-world for similar conditions. So if you can guess how, when, and where a reference photo was taken (or better yet you took it yourself so you know all those details), it will help a lot in duplicating the look. Yes, you can use extreme settings like 64 Bluesky Density, but this is really not a good way to get a real-world result because it's a very high value and will not respond in expected, realistic ways.
One thing I see a *lot* in reference photos is that the exposure, focal length, and post processing are totally unknown to the person trying to emulate them, and without this information it's very hard to know how and where to start. You need to make educated guesses and get good at analyzing images. Often the colors have been adjusted (I would not be surprised if that is the case in your reference for example), or the shadows lifted, or it is taken with a wide angle lens or telephoto, or even a long exposure in near-total darkness that looks like daylight, and people don't think to try to emulate the same approach in TG. But that is exactly what you want to try to do in most cases.
Take a look at this image for example:
http://www.steves-digicams.com/knowledge-center/kcenter/exposure2.jpgThat's a long-exposure, which you can primarily discern from the smoothness of the water. In reality that dock in the foreground was probably actually quite dark to the photographer's eyes, but with a long exposure you gather more light on that foreground object while also smoothing out the water and getting some blurring and apparent motion in the clouds (plus more color and light in the sky). In Terragen there is no "exposure time" (though there is a "blur length", which only works for *camera* motion), so you don't get blurring of e.g. water and other moving elements, but the exposure *brightness* effect works exactly the same way and should be used as such. I doubt most people even use exposure, it's overlooked! But very important just as in the real world.
- Oshyan