Breguet

Spangles - Dr. Tabby
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Hi, pure gold (24k) is too soft to work with, so cases tend to be made of 75% (18k) gold alloyed with other metals.

These other metals are what give the gold its necessary hardness, and also affect the color--leaving us with a shade of yellow, honey, pink, red, white, or grey (although older white gold alloys were sometimes plated with rhodium as well).

Different alloys have different properties, and while now there are some alloys that do not tarnish at all, in many cases while the 75% gold will not tarnish, the other metals in the alloy can and do. This is why a case can change slightly over decades. I think it's beautiful, personally, and I would never polish one of the old Breguets I have which has had its yellow color deepen over time.

In any event, looking at the alloys used by watch companies is very interesting. I am a fan of gray gold with 75% gold and 25% palladium as these are both "noble" metals which do not tarnish, so this is what my and my wife's wedding rings are made of. In various mythical traditions, old and new, differemt metals are held to have symbolic meaning and that can also add interest. For example, in a school of Daoism, gold represents that which we wish to remain untarnished, while platinum represents our pure aspirations. Meanwhile, a metal like silver which oxidizes represents letting go of things which wish to be rid of--a process of purification.

One last thing is that there are ways to make the same alloy harder than it would otherwise be, and that's through cold forging, a process which uses a lot of pressure and less heat. This is what allowed Charles Frodsham to use 22k (91.67%) gold in their case and Credor has used this process with their 18k rose gold Eichi II to allow the case to receive a higher level of polish to nice effect.