I’ve mentioned before that it bothers me when people say that all shojo art looks completely the same. However, I have noticed that when shojo gets animated, the main female protagonists tend to take on a homogenous appearance that wasn’t so present in the original manga. Part of this is due to the simple monetary and time constrictions anime face, which causes the character designs to become simplified from the original manga designs. Thus, while in the original manga the heroine may have frequently changed hairstyles (such as Kodocha), in the anime they are usually only shown with one ‘trademark’ hairstyle. There are many shojo heroines who have the same reddish-brown hair and amber eyes, and it becomes a lot more prominent in anime form.
The use of this color combination holds true even when the heroine has been shown to have a different hair or eye color in the original manga.
I’ve heard that black is hard to use in animation, so maybe this is why reddish-brown hair has become the ‘standard’ for shojo heroines – it’s realistic without being boring. And truthfully, I think Miki looks better with red hair, if only because the hair color looks better next to her blond boyfriend. The manga artist must have agreed as well, since later pictures of Miki depict her with lighter hair.