I don’t know why it changes so much… But I have noticed that Yarrow (Achillea millefolium) changes colour depending on where you plant it.
In my Garden I have 3 Yarrow plants. Before I carry on here its worth pointing out that the 3 plants all came from ONE plant that I took from a park after splitting the plants in late Autumn. So really there should be no difference in flower colour yet. But when my plants all came up the very next year I got 3 different colours, I was more than a little confused to say the least.
The plant on the left is almost white with a gentle tinge of yellow. The one in the middle is a kind of light, washed out pink and the final one is a definite pink colouring. The 2 on the left are both full sun for most of the day while the one of the right is only gets indirect light as its shaded by the fence you can see.
The only other difference would be the acidity to the soils where they live. Could it be that, like Hydrangeas, they can be tweaked to give a differing colour? I dunno. Googling gives no good answer other than certain PH value is needed for plants, which is obvious.
The only real difference is that the plant on the right has NEVER been able to hold itself up while the two on the left only need penning in so they don’t flow over and in between other plants. Not sure if that’s relative to the colouring but its obvious that sunlight plays a real part in the plants growth.
This year I may well split the white plant and move some of it to a different part of the garden to see if that one changes colour too!