Grey was more difficult to get consistent results (and therefore expensive). The danger was that you could 'lose' the highlighted area and couldn't predict what the finished product would look like.  The artist could in theory do a half tone on the original art with stippling or some other method but that means a slower rate if you are getting paid by the page.  The inconsistency of greys was one of the reasons why they switched the Hulk from grey to green early on and is probably the best known example.

Really dark colours also were problematic because they ended up looking indistinguishable from each other -- and became a muddy black.   If that really dark blue (IMO) that Ditko wanted to convey for Spider-mans costume would end up black anyway then it was easier to cut out the middleman and start with black and go backwards from there.  Spider-man transitioned to a regular book very quickly -- it's pretty well established that the content of ASM #1 and 2 were originally intended for subsequent issues of Amazing Fantasy -- and you can see once the ball started rolling on Spider-man that Ditko opened up the blacks to speed up the process.  The black with blue hightlights transitioned to a lighter, easier to depict solid blue pretty quckly.

(an interesting aside, Ditko went back to the black-with-blue highlights in ASM #28 for Spider-man's fight with the Molten Man *in the dark*)

The timeframes for the early Hulk comics in 1962 and the transition from grey to green are very close to when that final issue of Amazing Fantasy was being produced.   So if something wasn't working, why would you try it again elsewhere?

****

My wife does this to me ALL the time....HA.

...who would have answered "Black and yellow."

These people don't seem to want to get it right; they want to validate THEIR answer as the right one. They might have gone through Marvel creators until they finally got one who said, "White and yellow", and then claimed, "But Marvel said that's how it should be!"

Retards.