Oh, it's always so tough to tell, but you were smart to start them indoors early.

I would do a smell test, which works better with muskmelons than with watermelon. If it smells like watermelon it might be ready. Muskmelons give off that muskmelon-y scent when they're ready; watermelons are not so easy.

Does it look like it will detach easily from the vine? If so, there's a chance it's ready.

I can't tell which zone 5 you live in, but with generous heat (which melons love) and great soil conditions, they just might be ready a few days early. It couldn't hurt to leave them until they hit the 85-day mark, but they are looking great to me.

I am not a watermelon expert and, hopefully, those better acquainted with them may be able to be more precise in their answers.

As far as the light color where the watermelon rests on the ground, some watermelons are genetically prone to that, and some are probably like that because that area doesn't get exposed to sunlight.

Still, they've got to be close to ripe. When they are enjoy them!

All the best!

Can't see any pictures. But, there is a little curly vine right next to the melon stem. When it shrivels up and dies, the melon is usually ready to harvest. Also, turn the melon over...the belly should be a creamy, light yellow color when it is ripe. Lastly, but, least importantly, some people 'think' they can tell when a watermelon is ripe by thumping it with their finger. If it makes a hollow sound, those people say it is ripe.
Harvesting
Many home gardeners experience difficulty in determining when watermelons are ripe. Use a combination of the following indicators: (1) light green, curly tendrils on the stem near the point of attachment of the melon usually turn brown and dry; (2) the surface color of the fruit turns dull; (3) the skin becomes resistant to penetration by the thumbnail and is rough to the touch; and (4) the bottom of the melon (where it lies on the soil) turns from light green to a yellowish color. These indicators for choosing a ripe watermelon are much more reliable than 'thumping' the melon with a knuckle. Many watermelons do not emit the proverbial 'dull thud'when ripe. For these, the dull thud may indicate an over-ripe, mushy melon.

Please see the link to learn all you ever needed to know about watermelons.

Here is a link that might be useful: University of Illinois Extension: Growing Watermelon