Is it true if you put salon in hair color it want make your scalp itch?

Because I have a client that says the color burns and itchs and she has gray hair so she has to cover it up.
I heard that salt helps, is this true?

Salt won't help. There's some evidence that Sweet 'n Low helps.

Ohhh OK, gah maybe that's what it was. Thank You!!
Any other ideas?
And is there a amount I shuould use?

I throw in two packs when I do my wife's hair. I don't really know if it works or not.