I think aging plays a big factor in the Haunted Heart pictures. Mine fades after opening.
While I agree that temps can change a iris' color, I think that shade versus sun and type of camera also are big influences. Type of soil and minerals within the soil do affect brightness, but that would not be a factor in the same iris.
An example of how lighting plays a factor can be shown in a few pictures I took of Burseen's Virtual Reality.
Early morning, just opened:
In full shade:
Shade makes it look like an entirely different iris, but it is a bud from the same stalk as the other 2 taken within a 3 day period. A bloom that opened a couple days later was a shade of color between the first one shown and the dark shade one. The dark shade one did have some frost damage and did stay a bit darker than the other blooms. So in this case you could say that both lighting and temperature affected it's color.