Rav Yisroel Reisman (Ki Savo 5775) mentions an interesting concept for dipping apples in honey,
The Mechabeir says that a sweet apple is typically red. I have seen those who wonder of why we use red a sign of blood or of sin as a Siman. But once again, the apple is white on the inside it is only red on the outside. The Shana Tovah Umisuka is never purely white. It comes with some redness in it. We have to understand that the sweetness of life comes from that, from working, from fulfilling a Tafkid which is to overcome things with Mesiras Nefesh.
Where does the Mechaber talk about sweet apples typically being red?