Okay, let's totally ignore other possible light sources (reflections from a nearby wall on a house, etc.). Now there are only two sources of light outside. One is the sun, obviously (if it's daytime and the sun is shining). Where a petal has sunlight falling on it directly, that petal is NOT in shadow, and receives the (yellow) illumination of the sun.
But, if a petal is shadowed, that means NO sunlight is falling directly on it. This can happen because it's being shadowed by another petal, or just because that particular petal is facing away from the sun, right? Here, Ms. Wolf is speaking only of the petal facing away from the sun. The sole source of illumination on this petal is therefore the indirect light of the sky (which is blue, on a cloudless day, whether you've learned to see that yet or not
Of course, when a petal is shadowed from the direct light of the sun by another petal, if that second petal is colored (not white), and is somewhat translucent (rather than opaque), the light passing through the shadowing petal onto the shadowed petal can also be colored by the petal -- just as if it were passing through a filter!
But that's not what she's referring to in the specific text you quoted. She is only referring to petals lit by the sun (directly) and petals shaded from the sun (and therefore, lit primarily by indirect, blue, 'sky' light.
It sounds as if she's referring to a situation in which the sun is low in the sky, btw. Is this morning/evening condition? It doesn't matter, though. Go look at any surface in your yard, even blades of grass: Some are directly lit by the sun, and others are not. Right? The ones that are NOT lit by the sun are a bit bluer. I understand that can be hard to see, but you can surely see the opposite: The ones directly lit by the sun are yellower, a more 'yellow-green' than the shadowed green of the other blades of grass.