I'll take a shot here although you are likely to get all sorts of replies for fellow hookers.
1. Intake compressor blades are steel. They pretty much stay the same color as engines get washed often.When they are dirty, they get a little black but probably not enough to notice even at 1/35 scale. I'd say go with a flat dull steel. The intake housing is (outer and inner walls of the intake) are dark gray. By rear fins, I assume you mean the visible portion of the turbine inside the exhaust cone. That's a tough one, I will have to look closer at work and get back to you.
2. CH-47A, B, C and D's with 712 engines have a painted dark grey compressor housing. This includes the engine from the intake back to the combusion chamber (the larger round cylinder just forward of the exhaust). The combustor is a light steel color when new but heat quickly makes it darker and dull. Some even get the blue tint from heat. Its hard to explain, I have some pictures that I can scan and post maybe this weekend.
3. The exhaust cone starts as a shinny steel with a gold or brass tint. Over time they turn to a dull brass color (again pictures). The age uniformely for the most part. Atleast the part visible outside the cowling. Some get that bluish tint up at the engine end. They do not turn black from soot inside.
4. The gear box covers (we call them nose boxes) are painted the fuselage color. One point here is that early A models didn't have the intake screens. Later A's and B's had the screen only in warmer temperatures. They were romoved in the winter. The later screens (mushroom shaped) stay on all the time but have the rear panels removed in cold temperatures.
I'll dig up some pics, I know I have some that will help.