Peter - I can't see a timeline; I think your record is the first record for this site. Perhaps someone can confirm? I have difficulties bringing up the distribution.
Derek - I remember noting that at the time while that nearby appeared normal. I wondered if it was to do with the conditions as it was also a bit stunted.
The yellow colour might just be some natural variation. It might also be a function of the flower ageing, a response to nutrient deficiencies or a response to receiving too much/too little water.
I would exclude 1. ageing - given that nearby specimen which appears of older age has the typical colour; 2. nutrient deficiency - given that the surrounding vegetation appears well-nourished and (1); 3. too little water - given the amount of rain we had and (1). It might be too much water but this would have an effect on close by specimen too? colour variation of wildflowers based on mutation is not uncommon - it requires only a change of one locus in the DNA. In fact, these mutations are used by the horticulture industry to create variants.