The blue with black engine bay is simply a cost cut. It's cheaper to paint the engine bay black. Same as a silver one. They have grey engine bays. It's just a base colour that's applied first. As for the rust on whites, it's more noticeable on the whites. I've been a painter for 18 years now and I was sceptical on buying a falcon exactly like you are. Mine was under a year old when I bought it with 10,000km on it. Had the same signs of rust starting on all the very edges of the bonnet frame and engine bay seams. It only took me 1 night to fix it. Pull the airbox etc out for better access, find yourself some grey coloured Scotch brite and get some deoxidine. Then simply clean the rust of the edges and seams with it. Once you have done that, get a can of Wurth cavity wax. Spray it In all the seams in the engine bay. Let it set for a day. Then wipe off any excess with a rag and some wax and grease remover. As dust and dirt will stick to it. You want it just in the seams. 3 years later it still looks peferct.