its not color. it's considerably shorter on the sides, especially the lower sides. color has very little to do with it.
eta last unicorn: if he is dyeing his hair (which i'm not positive of, given how different it looks in different lighting), he's not doing it to only one part of his hair and and not another, he's doing it all over, and then cutting it differently on different parts. the parts of his hair we're discussing aren't sideburns, they are the sides of his head.
eta Rosebud: you're right, it's the cut. It's variation of the 'fade' (also known as a military cut). It's been more popular nowadays in hiphop culture. it's made by using electric clippers with, for example, a #1 attachment on the bottom, a #3 attachment further up, and either scissors or a much higher # attachment on top. A GOOD barber knows how to blend the difference between the #1 and #3 lengths so that there is less of a pronounced "line" where the one length of hair meets the other. bad barbers just cut the #3, then do the #1 from a certain point downwards. that's why color really has nothing to do with it, because once you get down to that #1 shortness, there isn't much hair to show the color. that's why I said he needs a better barber, also one that will do the fade all the way around the back (right now it looks like he kept his glasses on during the haircut
) Of course, he could just leave the sides the same length as the lower back, that would look good too
....i wonder if he's shaving it that close to hide gray tho....well if so he should do it all around
i know all this because i've been getting variations of this cut (ie the fade higher or lower down the side) for 15 or so years and i also do a bit of home barbering on my kid, and he loves getting 'faded' as they call it