When I applied for a job to pick watermelons summer of 1970 we were told, roll the melon over if the bottom is yellow pick it. If it is not yellow roll it back.
If your home melons set on the soil so the bottom gets no sunlight it will be white color on the bottom until it gets ripe then it turns yellow. Sometimes bright sunlight makes it hard to see if the bottom is yellow.
I like to toss dry soil up against my melons to block sunlight near the bottom to make sure bottom stays white color until melon is ripe.
When melon is yellow and ready to pick stem should be easier to brake off but I find that is not always true.
I have learned first day my melons turn yellow they have not yet reached maximum sweetness. Wait 1 more day before picking the melon.