I'm interested in filters, flash & lighting options, composition, and post processing.
Is it possible to get really vibrant colors or should I give up on that and focus on dreary, muted, or black and white photos when the sun's not out?
You can definitely improve on the standard "cloudy day" look with some preparation at shooting time and a bit of post-processing afterwards.
Switch the camera's white balance setting to cloudy: this will help keep the tones a bit warmer.
Compose to avoid large amounts of sky: you can play with the rest of the photo in post-processing but a flat grey sky will never look any better.
Try over-exposing slightly to brighten the shot up a bit. (All cameras do a different job of metering a scene, but if your camera has an exposure compensation setting, start by trying it at +1/3 or +2/3).
As always, be alert and be creative. Look out for glimpses of light as the clouds move, or look for more colourful subject matter (autumn leaves are a great example).
Learn to love the clouds! Cloud cover acts as a huge softbox, giving you a very even, diffuse light source. This makes cloudy weather ideal for certain types of photography, for example flower macros, where strong sunlight makes it impossible to capture delicate textures.
The two main things a sunny day gives you are contrast (bright highlights and strong shadows) and vibrant colour. So, to cheer up your cloudy shots:
Increase the contrast. If the big cloud soft box isn't working in your favour, some contrast adjustment will give your photo more punch. Hazy conditions give you even less contrast and tend to need a lot of help.
Increase saturation. Colours are always more muted on a cloudy day. Experiment a bit and you should find that you can make them more vibrant without going too saturated.
Increase brightness. Depending on how much over-exposure you tried at the scene, you may find the whole photo needs brightening just a notch or two.
Here are a couple of photos both taken on hopelessly wet, dismal days. The first has benefited from some increased contrast and saturation, the second from contrast and over-exposure. I appreciate they're both of a similar type, but considering the conditions at the time I was really pleased how they turned out.
You can have quite a dramatic effect by underexposing the background and using flash(es) to obtain correct exposure of your subject. Lighting setup should be the same that you'd use in dark for the same subject; of course, avoid spilling much light on the background, or it won't be underexposed any more.
If you're shooting film, you could choose a high saturation film, e.g. Fujichrome Velvia or Kodak Elite Chrome Extra Color.
Well, the first thing I would recommend is a polarizing filter. This really brings out the colours but it also takes some light ... about 2 stops.
I would recommend to try this first and then see if you need any more filters.
And remember for DSLRs a "circular" polarizing filter is required.
Overcast photography is one of my favorite lighting. Being based in the Nordic we got plenty of overcast days.
I wrote an blog post on the topic not long ago, I hope you'll enjoy it. http://guldmann.wordpress.com/2016/02/02/i-wish-for-a-cloudy-day/
anyone able to elaborate on ND filters and over cast days ?