Brassiness is a problem that everyone who lighens or colours their hair struggles with. You are not alone and you can fix this at home!
To fix brassiness, first you need to work out what level your hair is (how light or dark your hair is).
Use the picture below to work out your hair level.

Next, use the colour wheel below and find which unwanted colour you're seeing in your hair. Directly across from that colour is the colour which will cancel out your unwanted colour. You need to find products with this colour to fix the brassiness in your hair.
For example, if you're seeing unwanted yellow tones, the colour directly across from this is purple so any purple product will cancel out that yellow!

Now that you know your hair level, and the colour which counteracts your unwanted colour - you can find products specifically for you! Just look for anything at your level containing the colour you need.
For example, if you're a level 8 seeing a lot of orange tones you would look for a colour with a number starting with 8 which contains blue tones.
Level 9/10 can use any of the Wella Colour Charm Permanent Toners.