The Divine Mercy is a devotion exercised by many Roman Catholics and some Anglicans, to focus on the mercy of God, and can be traced back to the inspired artwork of a Polish nun, Sister Faustyna Kowalska, who was canonized in 2000.
When seeing the design for the first time, one might think the red and white represent rays of light or perhaps some sort of banner, ribbon or even the robe draped over Jesus by the soldiers when they mocked him.
But in fact, they are simple representations of the blood and water that flowed from Jesus when he was crucified. They are the fountain of spiritual life referred to in the teachings about the significance of the Crucifixion.
The image just shows the source of the blood and water and not the whole ocean of mercy; enough for everybody in the world. It is a simple image - almost childish - and that is because even though the magnitude of God's mercy is incomprehensible for us, the unconditional love that God showed to us through the sacrifice of Jesus, is childishly simple to understand.
See the meaning of the cross