What he said.

 

OEM brake pads are horrible about break dust! Especially if you drive with a little spirit and have 4 - 65-0mph stop signs every time you are coming to or leaving your house. A low dust break pad will do wonders for you (I'm using EBC Red Stuff for DD).

 

Looks like water mixing with brake dust in the rotor vent passages, as the rotors spins it all comes flying out onto the rims. An electric leaf blower will get the excess water out of there.

 

And the answer is: Rusty water! Unless the brake rotors are plated everywhere except the braking surface, when they get wet, (cast iron) it creates surface rust especially in the heat vents. It gets deposited on the inner rim inline with the vents.

 

I use this to dry everything.