found this on the web, an answer to a similar question to yours:
i am a hairstylist. All that "wait 4 weeks.... etc" is myth. The reason it is said is so you don't damage your hair. The hair's acidity level changes with color, so it needs to be restored. You can do this with a deep conditioner and simply re-color your hair, as soon as that day. Of course it's different if using bleach, but I understand you are simply using a permanent hair color. Go for it. I'm telling you, it is fine. People know that hair needs some time, but not why. (acidity level), which you yourself can stabilize that day. Good luck!!!