The Cokin Z-PRO 039 Warm (81Z) Resin Filter controls the bluish coloration that affects daylight film. It can also remove excessive blue from the effects of electronic flash. Good for general scenics when the photographer wishes to remove the bluish cast from open shade or just add warmth to the photograph.
It's Autumn. The clouds are low hanging and the sky is "patchy blue". You've gone for a ride upstate to the mountains and the light in open shade is bluish. A photo taken of your girlfriend with daylight balanced film leaning against the side of a barn will yield bluish tones that she might not find particularly flattering.
An 81Z filter (the strongest in the 81 series) will more than replace the yellow missing from the image and place all skin tones on a distinctly "warmish" level. You might even consider using an 81Z at the beach to make a hot scene even "hotter". The 81 series (81A, B, C, D and Z) allows you to choose how much warmth you wish to replace in the image.
The Z-PRO Series is recommended for focal lengths of 20mm onwards (in the 35mm format). This filter is made of CR39 Resin which is a reputable lightweight and shock-resistant optical material also used for ophthalmic glasses.