One way to think of it is just anything that wouldn't be likely to clash with other elements of decor when color trends change at a later date. It can be dark, medium or light, but not intense enough to draw the eye as the focal point. To me it would suggest something restful that works well in the background rather than making a dramatic statement. The lack of sharp contrast within the quilt seems important.
Just for some examples that are handy for me to post, if you go to this ebook link and then click "see more images", numbers 7, 8 and 11 seem like the kind of quilts I would consider neutral:
http://www.shopmartingale.com/strip-smart-quilts.html With just a little more contrast within them, the same patterns could be much less neutral. It seems to me that neutral is hard to achieve without a quilt being more of a blur, but I liked those designs.
I thought I bookmarked one recently that would also fall into this category, but I can't find it. As I remember, it was made of just 4 very large triangles or trapezoids in solid colors, a grayish pink, a beige, a gray and a pale, grayed out green. It looked soft and serene, but still had a distinct pattern.