Why red petaloids on the white flowers on a red-flowered tree?

Joseph Lin sent me these photos taken by Emily, one of his Green Club members. She was asked to photograph the flowering trees at a particular location, and included with her photos is this red crab apple that has some white flowers. When our ornamental cherries have white flowers, they are on a branch growing from the rootstock that is a white-flowered tree. In this case, I can't tell if this is on a branch from the rootstock, though there doesn't seem to be an area of all white.
In any event, I wouldn't have expected the the petaloids of the white flowers to be the red colour.

What is going on here?