I must agree with the others above and in the previous thread who say that this brooch has only ever contained an emerald when worn by the Queen Mother.
As has already been stated, when comparing photographs of jewels we must take into account how lighting can greatly change the appearance of a stone and how quickly that can change when the jewel is moved even a millimeter.
Another factor we must also consider is how vastly different photographs can appear depending on the camera and the way in which the photo is processed. Compare these two photos from HM's 94th birthday taken just moments apart but by different photographers:
In the first photo the color of the emerald is very apparent, yet in the second there is almost no indication of green. Here's a side-by-side comparison:
However, even when I edit the photo to adjust the color, the green is more apparent but the stone still appears quite different due to the way the light was hitting it. This is why both lighting and photo processing must be considered when comparing colored stones in different photos.