Hi, the colour filters all lighten their own colour and darken the complementary. Of course, in b&w all this does is give a tonal shift. The "sky" filters, yellow, orange and red darken blue skies. yellow is quite mild in effect (I'll ignore the fact that there are different strengths of yellow filters available for the sake of clarity), orange gives a stronger effect, while red will give almost black skies. A green filter will lighten green and darken red and could be handy for flower shots to separate red petals from tonally similar green leaves. My own favourite is the yellow-green which gives the sky effect of a yellow but also gives a longer tonal range where there is green foliage. Hope this helps, see the b+w filter handbook from Schneider if you need more info.