On Twitter a cat owner uploaded a picture of her tabby cat with this caption: My cat got shaved for sterilization but new fur become different color XD. I have changed the wording a little but the photograph is interesting:

So why is this? The first observation that I have is that the tail is the same color as the shaved portion. Does that tell us anything? Well it does, as I have just figured out why this has happened, as I write this.
This is a Siamese-type cat. She carries the Siamese gene because you can see the pointing (darker extremities). The pointing is caused by the extremities being cooler than the central part of the body. This is because the gene that controls the production of pigmentation is temperature sensitive.
“Cs is the Siamese factor and is much more temperature sensitive” – Sarah Hartwell
The tail is grey (darker) because it is cooler than the main part of the body. The shaved portion is cooler because it has been shaved and is not protected by fur. As it is cooler it is darker. It is like a pointed part of the cat.
As the fur grows back the color will revert to the normal red tabby because that part the skin will warm up. The pigment producing cells there will stop producing the darker fur color (probably due to the pigment called melanin).
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omg thank you so much for the information Michael
Good observation Michael. This explanation sounds plausible to me. ?