Shade is too light

I used Color Oops to remove all of the layers of old color on my hair, as I am quite a color chameleon and have made more than a few changes in the last six months.  I decided to jump on the "red kick", as this is one of the few shades I haven't had in a while.  I used Wella Color Charm 6R Red Terra Cotta w/ 20v developer.  While it was processing, because of the porosity of my hair after using Color Oops, I decided to wash out the color after only 15 minutes.  The color "appeared" to be the shade I wanted.  However, after I came out of the shower it was apparent that I washed the color off too soon.  Instead of the rich level 6 red I'd hoped for, I am instead more of a Level 8 Orange-Red shade.  It's very even and a lot of people have complimented me on the new color, but it's just not the shade I wanted.  I prefer to be a "richer" red (like the color I chose  - 6R).  My question is:  can I re-color with the same shade and  simply keep it on for the correct amount of time in hopes that it will darken to the shade I want?  I don't have any roots as of yet because I only colored a week ago.  A friend mine suggested a demi in the same color, as it will be less taxing on the hair.

 

Thanks in advance for your suggestions!