Can i put wella toner on white hair that has been toned with different brand

Don't bleach all over your hair again if it is already light enough that it's toning white, you would risk it starting to break off if you process it more! A lot of people here don't use the Wella toners as they have to be mixed with developer which causes cumulative damage (most people here tone with direct dyes diluted in conditioner)  so I'm not sure what to advise about that! Just wanted to let you know that bleaching the lengths wouldn't be a good plan!

It's not only that they have to mixed with developer, they also have a lot of ammonia so they're really damaging.

You don't need to bleach out the toner, that's the worst thing you can do. If it's not faded naturally, they all do just some fade out completely, others won't, then use a colour remover to take out the rest a few days at least before you bleach your roots. Then you can use the T18.

I'd strongly suggest using purple shampoo or violet dye diluted in conditioner instead though, your hair would really thank you for it.

Thanks ladies. Would the purple dye in the conditioner give me the icy look i wanted with the wella?

If your hair is very light then purple shampoo will counteract any hints of yellow, making it look cooler toned and more white.