Going to be a lot of Avengers but #144 is the one I thought of first...

A favourite issue, but I think the cover actually would have worked better with a more conventional colouring scheme rather than the red shade they decided to use over everything for some reason.

Shooter's first Legion issue (he does the interior art too, with finishes by Moldoff; the cover's by the fabled Swan-Klein team).

Yes, it seemed like the cover's creative license with the coloring was used in order to heighten the suspense (so readers wouldn't know she was wearing a blue FF costume and thus officially joining the team)...even though in the previous regular issue #80's lettercol, the coming attractions announced the next issue's story was called "The Replacement!"  At any rate, a favorite cover of mine too. 

I imagine that the idea must have been to highlight the title character, "The Exquisite Elemental" - and she does stand out, all lit up in gold against a background of almost solid red - but I think the effect would have been sufficient with everyone coloured normally just by her pose and centre position.

Yes, it's used for dramatic effect and she stands out....but also, remember, Crystal's go-to color/costume up until that time was yellow (the Inhumans costume), and even when she stopped wearing the Inhumans uniform (circa FF #66/Annual #5) and started wearing civilian clothes she often wore yellow civvies. In short, back then her trademark color was yellow. So the color scheme for this cover naturally played on that, with the idea that at first glance it would look like she was still a member of the Inhumans "team." Of course, open the issue and there she is on the first page, all decked out in her new blue FF costume. 

Fwiw it was similar to how they colored (some say deceptively) FF #62's cover; they wanted Reed's savior to be a surprise so they didn't color said savior green (as he should have been); instead they gave him a silvery color. All for dramatic effect, sure; but also so that the story's "surprise" wouldn't be given away on the cover. (Btw at the time there were readers who complained about the #62's cover's "deception.")

And fwiw Marvel's Greatest Comics #63  reprints the #81 story and the cover's coloring has been redone with "everyone colored normally" (though they kept Crystal in yellow, because at the time of MCG #63's publication in 1976, she had left the FF and had re-adopted her yellow Inhumans uniform). To me the recoloring makes the MGC cover look , well, not exactly bad but not special; I much prefer the original #81. To each their own  
www.comics.org/issue/1243976/cover/4/