The sun rises in the east with a hint of yellow; it casts unflattering colorless light at its zenith and it sets with a warm amber glow. Neon accents cityscapes at night. All of which are cues to photographers on how to employ colored gels on set.
Gel filters come in four categories:
ND filters are used to reduce the output of lights, but block their output like a pair of sunglasses for your strobes.
Diﬀusion filters are used to shape the character of hard lights and can resemble frosted glass or a translucent shower curtain.
This class of filter is used to correct artificial light to a neutral number or match natural light and may come in 1/8 to 1/1 strength.
CTO: Color Temperature Orange converts daylight to tungsten or matches the setting sun
CTB: Color temperature blue converts a tungsten source to daylight
CTS: Color Temperature Straw will make daylight match the morning sun
Fluorescent Correction: converts florescent light to match other sources or daylight
These are the filters you were expecting to read about. They come in every possible color and allow you to bathe your subjects in myriad shades of neon.
In general, if you want to intensify the results of these filters you can reduce the amount of light that you shine through them. If you’re using a light meter and you would normally want a light to read 5.6 on the subject’s skin, try turning the light down two stops to 2.8 and see if you like the results.
While I have always bought 0.61m x 0.53m (24″ x 21”) sheets of each filter you can buy combination packs with a nice variety of colors that will reasonably cover your flash for a lot less money overall. I recently used the Elinchrom Color Filter Set of 20 21x21cm sheets (EL26256) ; and they also make a Color Correction Filter Set (EL26255) that has a variety of CTO, CTB, diﬀusion and ND filters.
In a pinch you can produce an ND filter by stacking a CTO filter and a CTB filter because they cancel each other out and just reduce the volume of light that can pass through their combo. Similarly, for artistic eﬀect you can gel three lights next to each other in primary colored filters like cyan, magenta and yellow, or red, green and blue; in concert they will create neutral light where they all overlap and a spectrum of colors in the shadows where they do not. This may take some fine tuning between lights for true neutrality, but making something that looks cool is pretty easy, as I experienced in the quickly setup shot above.