Southerns tend to have more faded patterns and their blotches can range from liver color to blackish to gray. There is so much variance in southerns. Also, southerns a lot of the times have no or almost no patterns on the neck.
The orange/red hue is awesome on northerns, and if she is appr. four feet, then that may be the color she has settled into. Northerns can change very much from babies to adults. I purchased a red northern male into 2004. He was THE most red northern I have ever seen in person. Now, the red is all but gone! lol......here is a somewhat recent pic of him. He did the opposite of what yours sounds like.
To my knowledge, locality does not play a factor in the red.
Referring to your post below about feeding....yours sounds a lot like my two 2004 northerns that are now 4 feet as well. You had mentioned that you are feeding her medium rats? Where I purchase my rodents, a medium rat would be way to big to feed to my biggest pits that are over 6 feet long, so what you are calling " medium rats " could very well be different than what I would be able to get. Just be careful my friend about feeding food items that are too big that could potentially cause regurge. My two northerns I mentioned are eating medium adult mice on the larger side.
Here is my " red " male northern.

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Genesis 1:1