If you have unwanted yellow tones (generally found in dyed blonde hair, or naturally grey hair), purple shampoo is recommended since yellow is the opposite of purple on the color wheel.
Don’t leave it on too long: I accidentally left it on my dry hair for 30 minutes. My hair felt quite frizzy afterward, and even the Joico K-Pak wasn’t enough to get it back to normal.
I found that the blue shampoo really helped me for the first 9-10 months of my gray hair transition. But towards the end of my transition, my fading brunette dye was SO orange that even blue shampoo barely helped.