As with a lot of things "Carnival", it's complicated.........
First let's look at your questions about the marigold colouring. The glass has a clear base (no colour) and would have been pressed, removed from the mould and then (while hot) sprayed with marigold iridescence. The "marigold" would not have been applied before.
The effects and varying colours that may show in the iridescence can be caused by variations in the temperature of the glass, as well as by the thickness of the glass (which would also affect its temperature) and of course, the composition of the iridescence itself.
We can go right to the top and quote from Harry Northwood to find out what effect heat had on iridescence. "Spray on the glass very hot for Matt Iridescent and not so hot for Bright Iridescent" (Harry Northwood as quoted in "Harry Northwood: The Wheeling Years": Heacock, Measell, Wiggins). The matt iridescent effect is often described as satiny, while the bright iridescence is what may be called "radium".
Chas West Wilson, in his book "Westmoreland Glass", describes an iridised sugar and cream jug where the jug had a silver iridescence and the sugar had a gold iridescence. His explanation is that "while both the sugar and the creamer were about 1000 degrees F when sprayed, the temperature of the gold sugar was slightly cooler than the silver creamerā€¯.
And then of course, the actual type of iridescent spray itself creates different colour effects. Ferric chloride produced marigold. Combine it with a second iridescent coating (perhaps a tin solution) and you'll get different colours yet again.
Imperial were renowned for their triple doping.....three lots of iridescent sprayings! A knockout effect!http://www.geocities.com/carni_glass_uk_2000/iridescence.html
for some info on iridescence.
And now let's consider your recent question about the apparent lack of iridescent colour on the rim. Tricky question
. I've looked at my three vases and I can see exactly what you mean. The sides of the vase are richly iridised but that top section seems to be clear(er). It could
have been done by masking that edge - but I doubt that very much. What I think
has happened is that the spray has been angled at the side panels, and the rim (which slopes in and is at a different angle) has not caught the spray. That could have been accidental or intentional. I actually think it was intentional. It's a quality item, beautifully made.