It is really good to know that you have decided to quite this bad addiction and it is important that you keep your self on this path what ever happen. I would also suggest you to have consulation with some deaddiction centre as this type of addiction is very bad and requires professional help.
Now regarding the skin color change that you see on your arm. In my opinion this can be some sort of infection which is causing the skin discoloration. Skin infection
at the site of injection are very common in addiction and this can be one of them. This may requires treatment with antibiotics.
Thanks and take care.