Usually you can just follow the exposure indicated by a meter reading made through the filter. You may want to bracket your exposures so that you have a choice of brightness in your pictures. Dark-colored filters block a lot of light. These filters can interfere with the focusing of auto-focus cameras. If this is the case, you must switch to manual focusing when you use these filters.
Most filters produce only one color.
But variable-color filters are available that let you adjust the intensity of the color from light to dark. Other variable color filters produce either of two colors or various shades in between-for example, yellow or red with shades of orange between the two extremes. Once you have a variable-color filter adjusted for the color you want, you can use it just like a conventional color filter. You can obtain still other filters that are similar to conventional filters except that only half of the filter is colored while the other half is clear. This feature lets you change the color in only part of the scene, like the sky.