dark brown wants to go light brown

As long as you don't mind a reddish cast, you can probably get to light auburn brown using single-process color with 30 or 40 volume developer. If you want a cool shade, you're pretty much out of luck unless you bleach it to somewhere between pale yellow and yellow-gold and tint it back to the desired cool brown shade.

You might be able to neutralize the reds a LITTLE using a drab-base permanent haircolor (green, blue, or blue-violet), but bright sunlight and halogen lights will give you away every time -- even if the hair looks cool in ambient, diffused light, the moment light shines THROUGH the hair, the red pigment in the cortex will show up and be VERY obvious.

To wit: bleach to yellow, then tint back with something like level 4 violet-base Clairol Second Nature using 10-volume developer, and you'll probably end up with nearly flawless cool brown. Try to achieve the same shade using green-base level 4 or level 6 Miss Clairol instead of bleaching and toning, and you'll likely be disappointed with muddy olive-brown hair that looks red against ANY kind of backlighting.

Be careful, don't underestimate how dark your natural haircolor is. I tried to dye my hair medium brown from what I thought was dark brown, but I don't have dark brown hair I have either Darkest Brown or Black Brown. Make sure your hair isn't too far off your target or you'll turn red and look like a brassy fool in the sunlight like me!

Vanzack,

Like you, I have naturally dark brown hair. I have tried for many years to color my hair to a light brown, and until a couple of months ago was unsuccessful at it. I now use L''''''''''''''''''''''''''''''''Oreal Preference ( Light brown), and although I have my hair a little red in the sun, it is the closest I have ever gotten to a light brown color without getting all the unwanted orange/red. The only problem is that I am not really sure if I look better with light brown hair or dark brown hair.

Vanzack,

I know this happens to me all the time. I decide to change my hair color and cut it, only to later realize that I looked much better before. But today I decided to keep coloring my hair to a light brown, and not to cut it anymore since I want to grow it to my bra strap. And this time I will be strong, and keep that goal in mind! Isn't life just hard sometimes?! Just kidding