In the Astro Boy movie they made the same distinction between Tobio and Astro. Also Tobio's hair is actually dark brown, while Astro's is black. Note that Tezuka drew Tobio's hair with the tuft hanging down between his eyes as a sort of third cowlick. Both Astro, and the movie Tobio have this feature as an unrealistic widow's peak. (A widows peak is a feature of the hairline growing downward on the forehead, it is NEVER that exaggerated in real life. In the movie Tobio looks like Eddie Munster!)


Rear view of Tobio on the left.

The difference in the way Tobio's and Astro's hair is rendered was Tezuka's way of showing the reader/viewer that Astro was a robot and that that Tobio was the human boy. To the characters IN the manga or the Anime this distinction wasn't supposed to be visible, in many cases (as in the movie) people didn't know Astro was a robot. Hamegg figured it out when Astro walked by his EMF meter.