Ilford produces twelve Multigrade filters that are numbered 00 - 5 in 1/2 steps, with the lowest filter number corresponding to the softest contrast. Multigrade filters are very easy to use: no complicated calculations are needed when changing from one filter to another.
Contrast control with multigrade and variable contrast black & white photographic papers can be achieved in several ways. One of the most widely used ways is through the use of multigrade or variable contrast hand filters.
A magenta filter absorbs green light and transmits blue;
A yellow filter absorbs blue light and transmits green.
In this way, high and low contrast images can be made.