Blue Conversion filters help correct, for example, the prevailing colors of tungsten bulbs or warm light. Lower densities produce subtle corrections in color temperature in order to reduce or eliminate certain warm colors that dominate the image.
Orange Conversion filters help correct, for example, the strong dominant blue of shaded places in sunlit exteriors or on cloudy days. They are great for naturally reinforcing a sunset or a landscape of dunes or autumn undergrowth with backlighting.
FLW / FLD filters help correct the light from fluorescent tubes, that emit a very characteristic prevailing greenish tint. They recreate a natural daylight or artificial light (tungsten).