Midnight looks cheap to me especially on the big 15" so much empty space around the keyboard, felt award to pick up in store too, not one handed like the 13" or 14". Screen felt wobbly/flimsy too.
My first impressions.
I personally find the opposite to be true as the dark Midnight finish on the 15" Air makes everything blend in taking the eye
away from the space on either side of the keyboard due to the lack of contrast.
With the lighter finishes, there is more contrast between the keyboard and empty spaces on the sides, so the larger side keyboard bezels look more noticeable on the Space Gray, Silver, etc., versus the Midnight (not as bad as the 16" MacBook Pro of course - though the speaker grills soften that space, somewhat).
As for the build quality, I find the 15" Air
just as solid as my 13" Air. I can easily pick it up on either side with one hand - I do this all day as it's usually used on my lap. There is zero flex even while holding it from either front corner alone, and the display and hinge are tight with no wobble. The display hinge also stays put while quickly picking up the MacBook while it's open.
There is also no creaking, or chassis flex of any kind, at least on mine. This may be thanks to the extra support and clips that Apple built into the base to accommodate the larger frame versus the 13", as seen on some YouTube videos.
As for the aesthetic of the Midnight, everyone has different taste of course, and this is completely subjective, but to my eyes the Midnight is the nicest, most distinctive and regal-looking of the bunch, with Space Gray coming in at a close second. I am personally over Silver and I am not a fan of Starlight as I detest anything with a champagne and or gold hue.