I have several postcards that use the same image so I thought I would research an interesting series of cards posted between 1905 and 1910.
All three postcards were produced by Blum & Degen, who produced millions of cards between 1895 and 1908. The first two postcards are later versions of the first “divided backs”, which are from around 1905. There are no markings on these cards but I’ve found collectors who’ve identified similar examples.
The images on these cards are clearly colourised from a black and white photograph. The first couple of examples are very basic with limited colours.
The Photochrome process of creating a colour image from a black and white photograph was used extensively by postcard manufacturers before colour photography was commercialised.
1905
The sender of this card refers to Castell Coch as “the Welsh Castle of the Marquis of Bute”. Lord Bute had passed away five years earlier in 1900.


1907
This postcard was sent from a mother to her son. It shows how postcards were used in their golden age. There was no important message to convey so it’s likely this postcard was sent simply as an interesting image or to an avid collector.


1910
Blum & Degen trademarked the name “KROMO” so this later card is clearly identified and numbered. (The image below is from another postcard and used for clarity.) I assume “KROMO” is derived from the name of the colourisation process, which appears to have improved. This final postcard is clearly a better image, with far more detail and added colour.



Sources
Thanks to Chris Reynolds who runs the Genealogy in Hertfordshire website. His information about Blum & Degen was invaluable.