Andy - The effects of these filters will be much more apparent when you have some white clouds to contrast against the density of the blue sky. With a featureless sky you could simply burn the sky down during printing, but with clouds in the scene you'll probably want them to remain white hence the use of minus-blue filters (yellow/orange/red) during the taking process.

Go outside when there's some puffy clouds in the sky and shoot with and without the filters, then PRINT the negs. I think you'll see the sky deepen progressively as you go from none to yellow to red, while the clouds remain white.

Also, I've found the #15 yellow to be more useful than the #8. It seems to give an effect halfway between no filter and a #25 (and the filter factor is 1 1/2 stops). The #8 is supposed to render hues "naturally" on panchromatic film (i.e. normal b&w film like Tri- X) so you won't see a big effect. For real drama, use the #25 (or #29 deep red) along with a polarizer. You'll get the over-the- top "moonscape" effect.

Cheers!