My suggestion would be to choose a demi- permanent color, such as
Wella Color Touch, Matrix ColorSync, and Redken Shades EQ. There
are more than that, but that should give the basic idea of what to look
for. The beauty of demi- permanents is they have no ammonia,
but still require a catalyst, so they do not affect the natural
pigment, but are able to still add color into grey. Since this is a male
client they are more cost effective and take less time than a permanent
color to process. Formulation is easy, depending on his natural level, and
the brand of demi-permanent that you choose, most manufactures tell you to
use a level of color 1 to 2 levels lighter, but lower than an eight. I have many
male clients that love the end result of this. The natural hair does not change
but the grey is blended, or looks like highlights. Since they usually come
every four weeks I suggest a touch up around the edges on the cuts
between colors, as that is where it would be most noticeable as it grows.
Depending on the demi-permanent color you choose,
the distributor you buy from should be able to give you additional guidance.
Hope this helps.