Is this a joke? Because her real hair color is obviously black/really dark brown, it's impossible for her real hair color to be brown and blonde. Ayumi's been dying her hair for a long time but when she started out with Avex, it was presumably the original hair color (black). Now, she's just been dying it light colors possibly because we're all used to it and because she refuses to show her roots. She's bleached her hair for so long, the darker dyes actually won't stick to it.

re: eyebrows - That's pretty true that one can tell the real hair color by eyebrows but it doesn't always work because alot of people dye their eyebrows now, and I've known blondes with darker eyebrows before ^^