Filters can be added to a flashgun in much the same way as they are attached to a camera's lens. They have the effect of changing the colour of the light projected onto a subject by the flash unit. Warm-up filters, such as 81A, 81B and 81C are typically used when photographing white-skinned people to avoid giving subjects a cold white appearance.
The effect achieved with a flash filter is somewhat different from that obtained by using a similar filter attached to a camera's lens. The change in coloration achieved with a flash filter extends only as far as the flash output is projected, and does not therefore influence the appearance of a remote background scene. However, a nearby subject will be changed in colour according to the filter selected. When a filter is attached to a camera's lens, the whole scene within the camera's view finder will be changed by the filter.
More exotic flash filters my be used to achieve special effects, such as giving the interior of a room a particular colour wash, or tinting a nearby subject in a particular manner without affecting the background.