The heart pumps blood everywhere around the body. It's easier for the heart to do down because it is down, and it doesn't have to work so hard to get up. It's harder to pump up because the heart has to work harder.
See, the red one (on the left) is the one that was down. The right one was white because that one was up, so it was harder for the blood to get up.
So there you have it. I ended up doing the typing for her because it was a little much to think about what to say and try to peck through the keyboard at the same time. The color choice was hers! :-)