The skin also detects vibrations, though if it was tied to any of the five senses it wouldn't really be a sixth sense, would it? ;)
As for the souls, they wouldn't have to be colored in order to bear recognizable signatures. Since it's a sixth sense, it could just as easily be tied to a feeling about the particular aura as it could the color of it. Having it be something less distinct than a specific color would also avoid the issue of there being only so many wavelengths of color to use and him running out of different colors for each person (and you also wouldn't have to remember which color each person was if you mentioned it specifically). He'd also be able to more readily recognize people he's familiar with than he would strangers.
And for the range, the haze kind of imposes its own limits. If someone was far enough away, they would be indistinguishable from the "background" magic of the world around them, and being unable to see most magic through solid objects would also impose its own range restriction. I think the biggest thing that would make it potentially OP would be the ability to see through walls at will, so once you take that out and refine it down to the core of what you want, it becomes an acceptable ability as far as I'm concerned.
As for the souls, they wouldn't have to be colored in order to bear recognizable signatures. Since it's a sixth sense, it could just as easily be tied to a feeling about the particular aura as it could the color of it. Having it be something less distinct than a specific color would also avoid the issue of there being only so many wavelengths of color to use and him running out of different colors for each person (and you also wouldn't have to remember which color each person was if you mentioned it specifically). He'd also be able to more readily recognize people he's familiar with than he would strangers.
And for the range, the haze kind of imposes its own limits. If someone was far enough away, they would be indistinguishable from the "background" magic of the world around them, and being unable to see most magic through solid objects would also impose its own range restriction. I think the biggest thing that would make it potentially OP would be the ability to see through walls at will, so once you take that out and refine it down to the core of what you want, it becomes an acceptable ability as far as I'm concerned.