Deeznuts:
What you are experiencing, is commonly reffered to as galvanic corrosion.
See here:
http://www.artmetal.com/project/TOC/material/corros.txt
and here
http://www.pemnet.com/design_information/galvanic.html
If your water goes from pink (aka light red) to green, which is at the opposite side of the color wheel, I would be VERY concerned, especially since it happened in the time frame that you specified.
The green color comes from the Aluminium, not the copper.
The obvious fix, is to change your Aluminium res, because it is single handedly the cause/trigger of the galvanic corrosion.
Another fix would be for you to paint the inside of your Alu res, with some kind of non-conductive coating. Since you probably got an Alu res for its cooling capability (right?) then you might consider a thermally conductive coating. Then again, if it's just for looks...
Beware: IF YOU COAT THE INSIDE OF YOUR RES but leave a tiny little spot exposed, that part of your res will corrode at a tremendously fast rate.
So far, your Alu res has been corroding over the entire submerged face, which is not noticeable.