The brows will be the biggest challenge because grey hairs are a nightmare to cover and nine times out of ten your lady will say she doesn't want to go too dark but she wants the greys covered up(obviously)This poses a challenge because ideally we would pop the tint on the brows for 7 mins and leave to develope and cover the greys,however this will give an unatural,too dark, block colour.
I would do a thorough consultation,then if for example the client wanted brown eyebrows,I would tint the brows,take off,then reapply until desired effect,then only apply the tint to the grey hairs,this way you ensure not to go too dark BUT that the greys are covered,It can be time consuming so I would allow for a bit of extra time booked out.
I always advise that it's better to go a little darker initially because eyebrows do have a tendancy to fade relatively quickly,especially if going somewhere hot or swimming in chlorinated baths a lot.
Blue mixed with black Combinal tint is the darkest and a lot of ladies like to have very dark lashes,though again a lot like a more subtle look so I would go black brown unless client wants very light(very rare)in which case go for plain brown.