The stamps are known as such because they were only printed in those two colours; lilac being used for the 1+1⁄2d, 2d, 2+1⁄2d, 3d values and dull green for the 4d, 5d, 6d, 9d and 1s.
The colours were used because the authorities were concerned that the existing stamps were being soaked off envelopes and reused, and they wished to use fugitive inks that would wash away if immersed in water; the only fugitive ink colours available at the time were lilac and green.
The designs featured Queen Victoria’s head in profile, with lettering in the corners.
In 1887 they were superseded by the Jubilee Issue, with a variety of designs and colours.
[1] Due to the fugitive nature of the ink, many surviving examples of the green stamps have lost their original colour following soaking to remove them from paper; the examples that still have good colour are the most sought after by philatelists.