Velvia was an interesting film for nature photographers (okay, it is still around, but not used so much now). It greatly enhanced colors and contrast to create snappy, lively photos. With digital, a lot of nature photographers want to duplicate that effect. The problem is that it is all too easy to oversaturate colors and not truly get the Velvia effect (which I think has become a dated look anyway).
I started thinking a bit about this from what I had seen in a nature photo contest for NANPA that I had been part of as a judge. There were a lot of really wonderful photos. And some definitely stood out for their brightly saturated colors -- too much so. The colors were overdone and did not look natural or anything like "nature." Now that it is fall color time, I am seeing some wonderful fall color shots ... and some way over saturated photos that look more garish than attractive.
I think you really have to be careful not to oversaturate already colorful colors such as bold fall color on a bright sunny day. It is okay to actually trust the color of the scene and not feel you have to "improve it." Now it is true that digital cameras do not always capture a scene perfectly. Cloudy days can be nice for fall color, but many cameras, especially if you shoot RAW, will create an image that looks like it was shot through a gray, dulling filter!
The answer to that is not simply increasing saturation or vibrance in the computer. That often gives garish and unnatural results. What is important is to first set your blacks and check your whites in your photo. For an image to display properly on screen or in a print, it needs to have a full range of tonality from black to white (unless you are photographing in fog or something like it). Digital cameras will not (for a variety of design reasons) give that full range for most photos, especially on gray days. Without that full range of tones, especially blacks and whites, colors will not display to their full "abilities."
The first of these two photos of fall in Arches National Park in Utah is as the image came from the camera. The second has had blacks and whites set, midtones adjusted and some local traditional sort of darkroom work (slight crop, dodging and burning or local controls in Lightroom) -- no saturation or vibrance adjustments have been used. There is plenty of color in the backlit scene -- I am trusting the scene and only bringing out its full range of tonality (getting rid of some of the dullness that a digital sensor will often give).
Setting blacks and checking whites is easy with all Adobe products. With Photoshop and Photoshop Elements, Levels is the tool to use (or Levels adjustment layer). If you press the Alt or Option key as you move the left, black slider under the histogram, you will get a black threshold screen that will show you both when you are getting blacks in the photo and where they are. Most photos need at least a touch of black (though a highly colored image will often only show patches of color where color is maxed out).
Press Alt or Option again as you move the right, white slider and you will get a whites threshold screen. Whites are really, really sensitive, so I typically move this until something just appears on that threshold screen in an important part of the scene (don't worry about areas that are super bright, such as the sun or bright reflections on water). I have some free videos on my website, www.robsheppardphoto.com, that show this in detail for Photoshop products.
I mostly use Lightroom for these adjustments now. The same threshold screens are available and they are a huge help for me. Press Alt or Option as you click and drag the Blacks slider in Basic to the right. That will give you the same blacks threshold screen. Blacks are very subjective but you usually need at least something there besides a white screen (that says there are no blacks).
Whites are checked with Exposure. Again, hold down the Alt or Option key as you move that slider. And again, whites are extremely sensitive. I sometimes find that I cannot actually use Lightroom's adjustment when the whites just appear and so I back it off some.
These two adjustments will have a big effect on color and its appearance. Now you can set midtones, from the darkest to the lightest areas that are not black or white, with the Tone Curve (my preference) or Brightness and Contrast. You may need to tweak Fill Light and Recovery.
Now you can adjust color saturation. I almost never use Saturation in Lightroom or overall saturation in Photoshop. These are heavy-handed tools that are like repairing a laptop with a hammer. I may try a little Vibrance in Lightroom, but be careful because it can oversaturate skies very quickly.
I find I get the best color control with HSL in Lightroom or individual colors in Hue/Saturation in Photoshop products (using those adjustments in Photoshop are also in free videos on my website). HSL is great because you can go in and selectively affect colors. Digital sensors rarely treat all colors equally. You may find that a yellow needs more saturation yet a red needs less. HSL lets you do that quite easily, plus you can tweak the color of a color (its hue) and its brightness (luminance). If you have not explored the targeted adjustment tool, check it out because it can really help your use of HSL in Lightroom.