I understood.
Koleston perfect (and illumina) uses pastel developer mixed 1 part tint to 2 parts developer. Visual development as you described. So lets be specific about what you want to achieve. (Or i can't really give you the perfect formula for this) btw, this particular shade has been experminted and debated by the most talented geeks in a thread called granny gray.
So what level is your new growth? Is it a clean 10? Or a 9 or what?
What level is the light part of your mids and ends? What kind of condition is the hair? What's the porosity like?
What level is the orange? A 7, an 8? Or is it even darker?
See, you need to address all of these factors. If you just use one formula on three different colors of hair? What will you get? If your formula is dark enough to neautraluze the orange, you'll have neutral lowlights all through tour gray hair.