I think your artistic friend is wrong! If you use only the muted colors, the quilt will end up very dull. I would venture to say you could even add in additional lights and brights if you want.
Arranging each color from dark to light could be very interesting. You actually did this with the first 4 color rows, but then it sort of died off in the last 3 color rows on the right (colors are more mixed there).
Personally, because there is so much gray in the brown you are using (which I love!), I would not use gray as one of the colors. All rainbow colors would work great!