Arucha

The Arucha were an alien species which stole items of practical value.

Biology
Arucha were described as being leather-faced and grey. The Arucha were known to hiss.

The Arucha could not see the colour blue, due to evolving in an atmosphere that filtered out the wavelength of the colour. The Doctor likened this to the inability of Humans to see ultraviolet or infrared lights. If blue paint were to land on anything it would seemingly vanish. (The War of Art)

Technology
The Aricha had battleships which were a gun metal grey. The ships had dark corridors and computers which could listen in and watch conversations on planets below and detect space and time machines. They also had landing crafts which had retro rockets.

They dressed in what was described as combat outfits. They carried blasters, sonic lances and translation devices. They also carried a sensor device that perceived the colour blue for them. (The War of Art)

History
The Arucha evolved on a competitive planet. They fought several sentient species for survival, wiping out all of the others, becoming the most acquisitive species in the universe. They wished to possess all that was valuable for practical use and had no concept of fun. They had knowledge of the Time Lords, and knew that the Doctor was the last of them.

A ship of Arucha arrived at the Paloma Centre and followed the Doctor, intent of stealing whatever it was he was looking for. The Doctor pretended he wanted a painting and gave it to them so that they would leave. Two years later, they would discard the painting on an asteroid. (The War of Art)