I don't understand why a yellow area indicates ripeness of watermelons
(which I have never grown and cannot offer expert advice for). Seems
to me a "yellow spot" would mean a place the sun don't shine (on).

Bob Provencher's post echoes my own experience with canteloupes. They
practically shout "pick me" when they're ripe; the color changes; they
become very fragrant; and the stem slips off with the slightest
pressure. And oh boy, are they good!

Yellow spot is only applicable to those growing dark colored watermelons. All
watermelons change skin color slightly as they ripen . This change is more
visible on the underside where the melon lies on the ground, On dark green
melons like Black Diamond, Kleckly Sweets, Tom Watson, Sugar Baby...., this
area will appear yellow. On the white type melons, Charleton Gray, Sweet
Princess, Ice Cream ..... It will merely show as an off white. The
browning(dying ) of the tendril is perhaps the best indicator for a novice.
Most varieties of watermelon will hold for an extended period on the vine so I
would err on that side when uncertain as to ripeness.