Never targeted Chinook but have caught them incidentally on the Clearwater while swinging for steelhead with an old traditional Walt Johnson pattern deep purple spey. From what I understand, large intruder style shank or tube flies in chartreuse and black are commonly used. Maybe a prom dress with metallic green flashabou.
Caveat: I have caught exactly one King on a swung fly. But I have read a lot of material about it
From other folks musings, it seems to me that getting to the depth where the buggers sit it a bigger deal than the fly. Though dragging the rocks does not seem to be required. I know Kings were reliably hammered last fall on a particular river on both large leeches (3"-5", black and pink, all red, chart/black) and egg patterns (I use the term "egg pattern" VERY loosely). Another common denominator in my research is flash on the big flies.
But other have hammered them in the past on Purple Wooly buggers. There are some good threads on this forum on this topic.
"My king" was caught on a orange, pink and purple popsicle (marabou fly) tied stinger style with a gold cone head.
I found an interesting article about BC Kings on the Dean that mentioned the Lloyds Moose Ugly. I have seen innumerable hero shots with flies that are either a Moose Ugly, or are using the same color scheme. Chartreuse rear and bright blue up front. According to the thread where I found this (also about the Dean), the picture below is the original dressing, tied by its creator.
The one King I hooked in A.K. in the mouth was on a comet style fly with a chartreuse marabou tail, red edge bright body over silver tinsel and a schlappen hackle to match tail. I didn't have him on for long, as he was quite large ( left standing in muddled disbelief).
Never fished the skeena but I dabble in chinookin' whenever I get the chance. I have had good fishin in glacial rivers with prom dressy type stuff. Chinook love flash and they love a big silhouette above em. I've caught em on about every color but if I had to choose two flies I would run blue and purple in the sun and black and purple with cloud cover.
I generally either fish a a big black/blue/chartruse intruder or Tube Snake or a 2/2.5" black and orange tube ESL. Throw on some T-14 and swing her low and slow. A little stripping/movement never hurt anybody either. Except those nookies......
Having been fortunate to have lived on Vancouver Island for 7 years and have been fishing it for the past 22 years, I have fly caught a lot of Chinook. My number one fly is a traditional Muddler Minnow #2. My next fave is a black egg sucking leech. Now, this only works if they are fresh in the river system. During the early stages, the will actively take a fly. After that, it becomes more of a consequential catch than anything else. I have also caught them in Alaska on brightly colored flies in red, orange and purple.
^ I think the above post brings up a good point. Catching Nookies on the swing becomes increasingly more difficult the older the fish get. They just loose the willingness to take flies quickly as they progress through the river systems, so I'd concentrate in the lower reaches.