I have data for Ascot back to 1997, and these are the brooches worn (and frequency):
Albert sapphire x 1
Amethyst flower spray x 1
Cambridge pearl with drop x 4
Centenary Rose (The Queen Mother 100th birthday brooch) x 2
City of London x 5
Cullinan V x 5
Flame Lily x 1
Flower Basket x 2
Gold Dahlia x 2
Jardine Star x 8
Sapphire Cabachon & diamond x 4
Pointed petal diamond flower x 1
Grapes with sapphire bow x 1
Shell x 1
Small squarish, blue stones, flower x 1
Trefoil x 3
Victoria's bow x 5
Wattle - yellow diamonds x 2
So I think the only one to come close would be the Centenary Rose, with it's pink central flower, which was worn in 2003 and again in 2005, as most of these are diamonds only.
The Williamson Pink rarely shows up as pink in press photos, and has not, to my knowledge been worn to Ascot (unless it was prior to 1997)