Blending or covering grey on men's hair on the head using permanent or semi-permanent hair color is probably more common than you think. You might be surprised on how many men get this done professionally or purchase a home kit, and how natural it can look.
If the guy wears sideburns or some type of facial hair, especially when short, this can throw off their whole look. Why? When hair is longer, you don't tend to see the grey roots as much. But because the length of sideburns or a stubble beard is trimmed so often, the roots tend to really show.
Instead of constantly coloring the hair, there are products that come in tubes of basic hair colors that look like a marker or mascara that instantly blend the grey. They simply coat the hair shaft as a temporary means. I haven't personally tried any of them. So I can't say how well they work or if they stay in place and don't smudge. But they're worth a shot as most of them are not very expensive. Buy online (compare prices) or check your local beauty supply store.