thanks guys. I have seen pictures where they do this, and the red filter makes the blue in the sky turn very dark, and the clouds brighter. It will not wok on an over cast day, the sky needs to be fairly blue.
Another great thing the 25 red filter is good for is portraits or glamour shots. It sucks all redness out of the skin leaving it blemish free and very nice to look at and the prints are wonderful. It does require a faster film as you need to open up 3 stops but worth it.