To a general question like this, the simple answer is "ANY COLOR FILTER"!
Example, two apples. One green and one red. In B&W, both will look the same grey color. There is no or little difference in contrast between the green and red apple. However iif you use a green filter, the green apple will be lighter and the red darker, hence "increasing" the contrast difference between the two apples. And vice versa of course.
However if you question is "how to make the sky more dramatic - ie, darker like storm clouds, then a red filter (and to a lesser extent, orange) will do the job.
It probably means single coated each side. I have the green, yellow, orange and red. All the cheap hoya single coated. I might consider get B+W ones if I intend to shoot slides. But they are quite expensive to buy in so many sizes.
From an "Aroma" brand filter instruction guide for B&W films:
Yellow K1 - The filter gives greater contrast in the picture, making the blue sky stand out a little darker and yellow and red objects brighter.
Yellow K2, K3 - Since the filters absorb more rays in the violet, blue and green ranges, the sky comes out darker than with the Y1 filter. They are good for pictures of gardens, grass and shrubs, and similar scenes. Greens, reds and yellows come out clearly when using these filters.
YA1/YA2 Light/Medium Orange - These filters are good for pictures of mountains, the sea, and similar scenes. They make red and yellow objects come out brighter, and green and blue objects darker.
R1 Light Red - This filter is especially for infra red photography
R2 Medium Red - This filter is also used for IR pictures. It produces stronger effects in making yellow and red objects brighter than do YA1 and YA2 filters.
Green PO1/Yellow Green PO0 - These filters are recommended in panchromatic type film outdoor shots to produce pictures that are similar to what is actually seen by the naked eye (???)