Tiny, tiny dots!
Don't try to spot out the white areas with a single large application.
Instead, fill in the white areas with lots and lots of almost imperceptible little dots of dye.
Keep the pens moving as you lightly touch the area over and over - a figure 8 pattern of small movements works well.
It is generally better to spot too little.
The pens work well once you get used to them - and if you apply the tone this way the pens will last a very long time.