Cyan Colour Compensating filters absorb red and are available in 8 different densities ranging from 0.025 to 0.50.
When your image has too much of one particular colour, specific colour compensating filters are available to adjust this specific colour. These colour compensating filters effectively act as light sponges, absorbing excess amounts of a specific colour in different densities according to which filter is selected. By selecting these different densities very fine adjustments in colour temperature can be made.
For technical information, please see this
guide to exposure increases required for each density.