St. John's wort.
The color of St. John's wort (Hypericum perforatum) is purple.
They've started to flower and I'll be picking them later this week, as I write this (about now, as you read this). That yellow flower (and particularly the flower bud) gives a dark red color if crushed.
Pic: Red color from a yellow flower. And that color is a surefire way to make sure that yep, the plant under your fingers is a St. John's wort, one or the other Hypericum. And you can use them all -- all that give that color, that is.
I haven't been able to get purple fingers from the rather large flowers of various garden species.
And in rainy summers, after a few rainy days, all I get from flowers (or even flower buds) is water.
In autumn, around September or so, the color fades as well.
But even given all that: this is what started me on herbs, back when I was knee-high. Because if red fingers from crushing a bright yellow flower isn't magic, I don't know what is.