I came across my filters today...I've got a pretty good stack of them. Back in my film days, I used them a lot. They don't get much use now that I shoot digital but I thought I'd take a couple of them out for a spin. The black and whites were shot using a 29 red...it's a contrast filter and makes anything blue quite dark. They also make a 25 red which weakens the effect and if you just want to darken the sky a little bit, you can use a yellow.
These red filters really make the clouds pop!
For the color shot, I used an ND64. ND stands for neutral density...it's like putting really dark sunglasses on your lens...lets you open things up in bright light or slow your shutter speed down. This is what they use in those pictures of waterfalls or rapid water and the effect is like a cloud or blur...pretty cool! Now where can I find a waterfall in south Texas?