Some oregano like Zorba red does have leaves that are a purplish red. If you cannot see anything that is causing the color change, it may be that the plant has just matured to it's true color. Look up your variety and see if this is the case.

As for soil--the best soil for seed starting and growing in pots is Pro Mix BX. It is the only thing I use in my greenhouse. Small bags are expensive though. I buy mine in a 2.8 cubic foot bale.

I don't know if you have it in the states but Presidents choice magic soil is good. I wouldn't throw out your miracle grow. Instead I'd mix it with a regular potting soil. The big problem with Miracle grow soil is that it has too many polymers that hold water and it's easy to overwater. Spread out over a larger area though it would be OK