Yellow Line? What Yellow Line?

I thought the whole reason for the yellow line on the top of a MLB ball park fence was to denote what was a HR and what wasn't. Simple math:

Above or on the Yellow Line = HOME RUN

Below Yellow Line = IN PLAY

If the padding at the top of the fence is just there to protect the outfielder, WHY IS IT YELLOW? Why not some other color, like mauve? Chartreuse?

And how much bad luck do the Padres have for Chase Headley to hit a ball that bounces off the top of the fence at EXACTLY the right angle to return to the field of play? Not bounce forward into the stands, not bounce straight up, but BOUNCE BACKWARDS to the outfielder?

Chase Field Outfield Fence, Mr. Headley and my Fantasy Baseball team will not be sending you a Christmas Card this holiday season. Thanks for nothing.