Like all color treated hair, sometimes you're missing the lustre or brightness if it's been a while between salon visits. I would never trust myself to use a box color or highlights on my hair, so for the longest time I would just kind of deal with the brassiness until my next appointment.
Then, all these blonde products started appearing on the market from high end salon lines to John Frieda in drugstores and I became intrigued. I asked around, did some research on reviews and have been using this method to revive my blonde highlights when needed ever since. I don't do the shampoos and conditioners for blonde hair for a few reasons. One, they aren't keratin treated hair friendly and two, even before I started doing keratin treatments, I never noticed a difference when I used those products alone.
One product that really hit a home run can be found at the drugstore and is the John Frieda Go Blonder Lightening Spray. This isn't your typical Sun In product (I know you 90s people know what I'm talking about). I've never had to deal with Carrot Top colored locks after using the John Frieda spray, it leaves no filmy weird residue on your hair and it works gradually to lighten your hair. It's also not a product you need to use all the time. I find when my roots are a little too grown out or my hair is looking kind of brassy, it usually takes two uses and I'm back to where I want to be. The spray mostly picks up on the highlights already in my hair and helps brighten them back up. If I concentrate on my grown out roots, the spray helps soften the harsh line of my roots, which helps disguise them, but it's never a drastic change at my roots.
To use, shampoo and condition you hair like normal. Comb thru towel dried hair, part hair where you normally do and spray all over your hair where you want it lighter. I usually just focus on my top layer. Then, comb the product from roots to ends to evenly distribute throughout your hair. Blowdry your hair completely and for best results flat iron after blowdrying. The heat styling is what activates the product and lightens your hair.
The second great product is the Redken Blonde Glam Perfect Platinum Conditioning Treatment, which is a purple-ish hued conditioning cream. It packs a one-two punch in that it helps correct brassy color and adds hydration back into dry parched hair. It has color depositing pigments to help lighten you hair and it doesn't have an overly strong hair color smell to it. If you remember Redken's Pure Pearl before they discontinued it, this is the replacement. The Pure Pearl is hands down my favorite blonde enhancing product, which is a huge bummer that it's no longer available.
Depending on what you're looking for, you could really just get the John Frieda spray (which is sold year round in my Target) for under $10 and spray it on after a great deep conditioner if you're trying to fight brassiness and dryness. The spray is hands down the easiest and there's no waiting. Spray and blowdry and you see results. I have a thing for instant gratification.