It's hard to tell for sure from the scanned images, but to me it looks like you are slightly underexposing. You don't seem to have a lot of detail in the shadows. I think this prevents you from printing any areas with a true black, and will keep your contrast looking low/muddy.
As far as filters go, others have given you good advice. I don't see that they would make much difference in the three images you presented. But for images where a contrast filter will give you a strong effect it's very hard to reproduce the effect in post (either scanning or printing). So you should certainly learn to use filters and use them where appropriate.
Yellow, orange, or red filter does NOT give you more contrast.... perhaps it is sometimes called that but all it does is to change the relationship between different natural colors - which might or might not result in increase in contrast. If you are shooting a scene that includes cloud, yes it does because the blue sky gets darker in comparison to the white cloud.
Looking at the photographs you gave as examples, I'd say no, don't use filters. Also, when one uses filter, he/she has a goal. I want THIS color to be darker, so I pick THIS filter. Putting one on neither (always) increase your contrast, nor improve your photograph.
If you use RED filter on your photograph with bricks, it will do funny things. Red will cause your brick (which is usually reddish) to be lighter because it will pass more red light which in turn will make that portion of your negative darker. Which in turn make your prints lighter, which in comparison with white sign and grout within the brick wall, it will be LESS contrast.
YES, there isn't a contrast filter per se only ones that darken the rendition of a particular color.
Originally Posted by tkamiya
Now if I could only find my wire filter. Really useful for eliminating those overhead telephone and power lines. :)
I usually use yellow but sometimes I use orange and red. You could use red and a polerizer to really darken the sky.
It depends on what you want to accentuate. Color filters merely change how the film sees the colors that are reflected from the scene.
Orange filter makes orange hues lighter, and to a lesser degree yellows and reds.
The complimentary (opposite) color of orange is blue, which means that less of that color will be let through to the film, rendering them darker. To a lesser extent this applies to purple and green.
So, color filters will increase contrast between the color of the filter itself and its complimentary color. But it does not directly alter the contrast of the negative. It just moves tonality around with some colors lighter and others darker than they would appear without the filter.
Film development alters negative contrast directly, regardless of what colors were recorded at exposure. Longer film development = more contrast, less film development = less contrast.
To some extent you can say that film exposure also alters contrast. If you don't give enough exposure some shadow values will be lost, which means that you lose contrast in the shadows, and it also means you have to over-develop which gives more contrast, so it's an indirect effect.
Don't confuse color rendition with overall negative contrast. A green leaf correctly exposed and processed with a green filter will look very light in tone, while one exposed with a red filter will appear almost black.
Look at the link above to see a picture of the color wheel, which further helps explain the reasoning with color filtration.
The choice of film is at issue as well. TMax has a truer color rendition than, say, Tri-X. Tri-X is more sensitive to blue light, therefore a yellow filter would be appropriate for darkening the sky value, bringing the scene more in line to how the human eye percieves it. That having been said (written), I still prefer TriX film for almost all of my photography. And yes, I use filters of all types, depending upon how I want the scene to appear in my photograph (aka "previsualization").
As others have said, there is no such thing as a "contrast" filter for black and white.
There are, however, filters that help you differentiate different parts of certain scenes from other parts - e.g. a yellow filter can help clouds stand out from a blue sky.
And filters that reduce the effect of UV light can also make a difference. The information above about the difference between Tri-X and T-Max when using a yellow filter is most likely due to Tri-X's greater sensitivity to UV light (which a yellow filter will tend to block).
From the examples you gave, a filter would probably not make a difference. Except when there are sharp color differences, a filter does not affect contrast. Development does affect contrast, and slightly longer development would give you more. You can also compensate in Lightroom.
A medium yellow filter is usually considered to be a correction filter rather than a contrast filter - it renders the light values in a daylight shot more closely to what the eye sees. It is generally a good idea to use one for general shooting. Contrast filters - red, orange, green, blue, etc. - lighten the appearance of colors similar to them and darken their complements. With a red filter, for instance, the bricks in your red brick building shot would appear light gray. They are quite useful in many black and white situations.
Yes. No. Maybe. It all depends what look you want.