A good rule of thumb is: a color filter lightens subjects close to its own color and darkens complementary colors.

Typical effects in black-and-white:

• red: Strong effect. Common for landscapes because it darkens blue skies and makes clouds stand out dramatically. It can lighten skin tones and make green foliage look lighter/ghostly or darker depending on the scene mix.
• orange: Similar to red, but milder.
• yellow: Subtle, general-purpose contrast boost; often used for a gentler sky darkening than red/orange.
• green: Lightens green foliage and can darken reddish skin tones, so use care in portraits because complexions can look blotchy.
• blue: Lightens blue skies and lowers overall contrast, which can enhance a hazy look.

Use by subject:

• landscapes: red, orange, or yellow for darker skies and stronger cloud contrast.
• foliage/nature: green to brighten leaves and plants.
• portraits: usually avoid strong green; red can lighten skin, yellow is subtler.

Film vs digital:

• film shooters commonly use these filters at capture.
• with digital, similar effects can often be added during black-and-white conversion, so physical filters are less necessary. However, some photographers still use them to alter RGB channel balance before processing and help preserve highlight or shadow detail in certain colors.