There's more than one "red beach" in Greece, a name given to any place where the sand is a bit pink or rusty-colored, and several of them are often naturist-friendly, including the Red Beach on Santorini. Some sources may confuse the different "Kokkinos Ornos," so be sure which island is meant when you decide to go.
This favorite beach in southern Crete can be reached by water via small boat, kayak, or, horrors, noisy, turtle-scaring jet ski or by hiking about 25 minutes from Matala. The access is up over a fairly steep hillside and one path, leading from a dirt road into the hills, requires scrambling over a small area of boulders.
The other path, which is easier, starts at the end of the row of bars on the south side of Matala bay where a steep stone-paved ramp leads up to the cliffs. While still steep at a couple of spots, it's not as sheer and most will not need to use their hands to scramble up. If you stray off the obvious path, be aware that there are some unexpected open-at-the-top caves in the limestone cliffs, some of them occupied, but in any case, not a place you want to discover unexpectedly when hiking alone.
At the top of the cliffs above Red Beach, about 300 yards or so from the beginning of the path up through the bars, there is a goat fence with a gate. Go through this, close it behind you, and begin the descent to Red Beach.
On the way down, you'll see large rose-like sea squill bulbs in their summer dormant state, just poking up through the soil; they will begin sprouting in late fall.
In high summer, there are sunbed and umbrella rentals at Red Beach, but most of the time you'll be relying on your own towel or sarong to keep something between you and the sand. The sunbed rental hut offers limited beverages, including a great kanalada, but may not always be operating, so bring the essentials with you. The whole beach is reliably nudist-friendly, but the most naked bodies are at the north side of the beach near the rocks which were carved into intriguing ancient Minoan and Egyptian shapes by a Belgian man named Gerard a few years ago.
On a cool spring day, full nudity is the exception rather than the rule, but there are still plenty of visitors in bathing suits. If the waters are rough, use caution, as there is no lifeguard.