The thing is, in your case, this is probably a better solution than the hair maintaining various tones.
Because your natural hair and replacement hair are not the same composition of colours, using something that gets absorbed into the hair will just alter the difference, not eliminate it.

You can of course apply it in streaks and this will cause a variance in the final result, but it needs to be fairly consistent at the points the system hair meets your natural hair for it to be a good blend.
So any streaking should be on the top, rather than the back & sides.

Re the Nair, as I said before, allow a couple of days/washes to really gauge how much hair has been thinned out.