Could that be sunburn, or is it a type that gets red with more sun? About the watering--I'm anxious to hear the replies. One of my hoyas is wrinkly and dry. I haven't watered it any less than my others, which are in the same mix, and it looks like this one may die. So I'm stumped. I water about every 4-5 days in summer and once every 7-10 days in winter. All my hoyas are doing fine but the one I mentioned. Do you use the fine orchid mix? If so, it dries up faster than other mixes, but is good for hoyas because it's light and greatly reduces the chance of root rot. You can get away with watering a lot more often.

The others look pretty darn good. Hopefully someone else will chime in here with some ideas.

Is that heuschkeliana? If so, mine redden a bit in high light, as well.

Just a note though, almost all hoyas (some australis types, pachyclada, and a couple of others are exceptions to this rule) should not be allowed to fully dry out between waterings. They aren't true cacti/succulents.

Heuschkeliana prefers slightly damper conditions than you are allowing it. I water mine when the top inch of media is dry, which for me (due to my climate and the potting mix I use), is weekly in the warm months, and every 10 days in the cold months.

Yes, it is a heuschkeliana. I have allowed it to become dry before watering. I well start keeping it on the moist side. Thanks.

A follow up note. The plant appears to be greening up nicely with additional watering.