Try an ISO setting of 200 next time for something right at dusk or dawn.
You have to get into your mind the triangle of shutter speed, ISO, and aperture. That's why I suggested earlier you set the camera up to use "standard" shutter speeds and f stops, WHY?
Because if you have "standard" f stops and shutter speeds, one "open click" of the f stop is the same exposure change as "one click" slower on shutter. 1/30 is double the opening of 1/60 shutter, so if you change the f stop by that amount, you HAVE NOT CHANGED the exposure.
The film speed / ISO works the same exact way.
All you did in that photo by changing ISO is to change the exposure. You could have done so by either ISO, shutter, or f stop
WHAT ARE the differences?
GENERALLY you want the lowest 'within reason" ISO for the highest quality photos
You want small enough aperture that you get the depth of field you need. If you get "too small" you can produce unwanted effects, you'll need a much slower shutter which might introduce blur and shake, and or faster ISO, producing noise and grain
You want a shutter speed fast enough which will stop action, or compensate for camera shake. If you lean towards too high shutter speeds, you'll need a higher ISO producing more "noise" and grain, or a wider aperture, reducing your depth of field
And all three have to be "in a balance" to give you the exposure you need.
To take some of the "science" out of this most of us, in some way or other use bracketing. You can set the camera up to do this, or just do it yourself, that is, take a shot, and chance the exposure "one up" and "one down" to be sure you "got" a really great one as opposed to a "good one."
Also, in shots such as sunsets, digital does not have the "dynamic range" the your eye or a film camera has. This means that you "look" at a sunset and you can see the lightest and darkest areas just fine. But your DSLR will either show the darkest areas as way too dark, or the lightest areas as "blown out" OR BOTH
WHAT TO DO about this? Only real cure is to learn how to frame the shot to mimimize the effect, or learn to use "graduated filters." These work just like the windshield in your car........the graded filter has a darker area at the top of the filter, and is more and more clear down at the bottom
So you can take a dark sunet, with the land area "blacking out," and the sky "blowing out," and hold the filter in front of the lens. You can move it down so the "blown out" area of the sky is somewhat lightened, allowing the blacked out landscape to show up