Film Gels are labeled by the "CT" name. If the gel is to push the light temp. to a blue colour, it will be "CTB". If the gel is to push the lighting temp. towards yellow/orange, it will be named "CTO". That's how to tell what colour gel you are buying.
Secondly, you then need to know about how powerful the gel is. The power, is labled by a fraction (1/4, 1/2, full). For example, if you have a tungsten (yellow light) , and use a CTB Full, the gel will shift the yellow fully to blue temperature. 1/4, 1/2, etc. gels will make minor shifts to the colour temperature of your light.
So in other words, if you want to change your Halogen worklight colour temperature to full blue temperature, use a full CTB. But if you want to only change the temperature, but still have a little bit of warmth to the light, use a 1/4 or 1/2.
Also, in using gels, the gels will make the light dimmer (by aperature stops). So if you use a CTB full gel, u will notice that you cameras aperature will have to be boosted by 1 full aperature stop.
Really, it is best to probably buy a couple sheets of every colour and temperature rating. Experiment with them, and learn visually what gels
REALLY do for you and what colour gel you use the most. This wil save you money in buying a colour temperature gel you wont use because its maybe too small a change or too much of a change.
I hope this helped!
