What Jonas sees in the apple is a bit of color. He does not really realize what he is seeing, though.
This happens very early in the book. All that we are told is that somehow the apple changed. When I first read the book, I had no idea what sort of change was going on at that point.
Much later in the book, the Giver will tell Jonas about color and Jonas will realize what it was that he was seeing. But at this point in the book, he has no idea what happened with the apple.
Jonas sees the color red in the apple while playing catch.
Jonas sees a flash of red in the apple. He doesn't know what it is, and is very confused, until he seeks The Giver, who tells him that he is starting to see colour.
Did you even read the book? The giver tells Jonas later in the book.