And I had to start another thread over in the Traditional Highland Attire forum to find a few answers. As it is so far and as best as I can tell:

The use of tartan on a non-traditional looking kilt does not bring it over into the traditional highland attire.
Also, the use of a solid color fabric on a traditional kilt does not, as Matt pointed out earlier, exclude it from being a traditional kilt.

That leaves the construction and design to determen whether a kilt is contemporary/non-traditional or is traditional Highland attire.

For example, my Buzz Kidder is not a traditional kilt, but is a solid color kilt. If I had a low, jeans type cut Freedom Kilt in tartan materiel, it would not be a traditional kilt.
There should be no problem with wearing either of these with accessories outside of the traditional Highland attire; for example, without a sporran in the front etc.