High carbon steel knives will develop a patina over time. This is part of their character and story, to keep a record of the wonderful meals they prepare. For folks used to a stainless steel knife it can be a bit of a shock at first but the layering of a patina is a magically adventure that can really bring you closer to your knife and food.
A knife's patina is caused by the acids in food interacting with the knife steel. This is not orange rust but a layer of oxidation that forms that will actually prevent orange rust in the future. The layer provides protection and aesthetic character to a knife. A rainbow of colors can appear depending on the food cut and type of steel in the blade. Eventually the patina will even out to a nice grey tone.
Some of our knives come with an etch or patina. Generally changes to the knife will be less apparent than on a patina-free knife but the tone will change with use. A patina-free knife will change rather dramatically when first used but over time become more consistent.
Here is an example of a forced patina with salsa on a 52100 high carbon steel blade. This is an extreme example and the salsa was left on the blade for an extended period of time. Patinas can be forced with many things including mustard, lemons, tomatoes, etc. Patterns in a patina can be altered with application as seen here. It is not necessary force a patina as your blade will develop it's own over time and use - but it can be fun to try and a good way to see how food interacts with your knife.
1. The clean blade
2. Applying salsa
3. A subtle patina starting to appear
4. The patina developing with prolonged exposure
5. Taken to the extreme
To stop the patina you can dip your blade in a baking soda and water mixture and then rinse off. The patina will still change on the knife with use. If you don't like a patina it can be removed carefully with some simichrome polish and then cleaned with mild soap and water afterwards. Have fun but proceed at your own risk.