I think that the brooch which can be seen in the image posted by Franck is one of the Cartier ruby brooches.
Part of Franck's image (posted to help people visualise)
The first point I think is important is that the image has been hand coloured and "touched up", so that it is not a 100% accurate reproduction of the original photo.
The ensemble worn by Queen Elizabeth the Queen Mother was in fact pink.
HM wore the dress and matching coat numerous times during 1947. (Some times she wore only the dress). The dress and coat are so distinctive that both are easily recognised in photos.
On all but one of the occasions, which I have found, QEQM wore either both or one of the Cartier ruby brooches. On one occasion she wore the Cartier Palm leaf (Paisley) brooch.
Proof that the ensemble was pink 1. HM wore the outfit for Gold Cup Day at Ascot
According to
Tatler HM's dress was pink and she wore the ruby brooches.
2. Among other occasions when QEQM wore this outfit was the return to the UK and the balcony appearance at Buckingham Palace after the South African tour.
All the numerous press reports state that QEQM was wearing a pink outfit. For example
Because the hand tinting in the image Franck posted was done to indicate a blue dress, the rubies show as a darker blue inside the outer ring of diamonds. The diamond elements above the central motif have also been made darkish. What looks like a shadow to the lower left hand side of the central element is all that can be seen of the second ruby and diamond element in the brooch. When I looked carefully, I could discern part of the lower diamond leaves of the brooch showing through the fern in HM's bouquet.
I was interested to note that the second smaller part of one of the brooches is not always immediately discernible in photos. In part that is because of the angle from which some photos have been taken, but possibly light exposure may also play a part. I suspect that in the image Franck posted it is the fact that the photo has been taken from the right hand side which has resulted in the second element appearing as a shadow.
I will post some more images of HM wearing the dress and brooches to give an idea of what I am suggesting may have given us a mistaken idea. The photos above of HM on the balcony of Buckingham Palace and at Gold Cup Day at Ascot show that the diamond elements did not always show up clearly in photographs.
Below HM is wearing a different dress and coat, but the photo shows part of the ruby element obscured from view.