Microscope filters are used for both observation and photo microscopy. Each microscope filter is used for a different purpose and all are typically placed in the light path. Microscope filters are used to increasing contrast, block ambient light, or to selectively transmit only wanted wavelengths.

Blue Filter: Typically used in a brightfield application. When low levels of light are used, you usually get a yellowing effect because the lamp is turned low. The blue filter brings the light back to a more natural white light.
Green Filter: Often placed in the light path to improve the performance of achromatic and planachromatic objectives. When using phase contrast the best images are often produced in green light.
Yellow Filter: Typically used when the microscopy images are blue. The yellow filter brings the light back to a more natural white light.