Brassy tones in hair are very common. They can be caused by a mixture of things;
Poor water quality when washing your hair
Hair colour hasn’t been lifted properly
They are sometimes inevitable however there are a few ways you can keep them at bay.
Getting a shower filter (I use blu)
Always wearing a hat when out in the sun to protect from heat damage
Ask your hairdresser if they can do a pre cleanse on your hair before your colour appointment (malibu or color wow have products that do this)
Keep your hair healthy by maintaining with regular moisture masks