They were initially colourless, but eventually they began to be embossed in rose and later vermilion. Sometimes the stamps were impressed on sheets of paper and cut out in order to avoid stamping each document individually - these were among the first adhesive revenue stamps.
In the 19th and 20th centuries, impressed duty stamps were used in many British colonies and dominions, including the Australian states. They were also used in a few non-Commonwealth countries such as Egypt.
A few countries such as Kenya, Hong Kong and several Malay states used impressed stamps as a means to cancel revenue stamps - in this case these are known as over-embossing dies.
Impressed duty stamps were commonly used throughout many countries until around the late 1970s, when they began to be phased out. Nevertheless, they are still in use in a few countries such as Uganda.
Anyway, here are a few examples from Malta, with the recorded dates of usage below:
1d and 2d values with imperial crown (pre-independence) and 2d, 4d and decimal 1c7 values with mural crown (post-independence)
(note: the image for the 1d is taken from Revenue Reverend, the rest are from my collection. There is an error since the earliest known use of the 1d is 1922 not 1926.)