True orange is a very rare color in broad leafed rhododendrons. Those listed as orange usually have some pink mixed in and would be more accurately described as peach colored. Those with orange tones and -10 hardiness that I can think of offhand include Hello Dolly (borderline at -10), Viscy, Amaretto, Hindustan, Orange Honey. None of these are going to be easy to find now that the number of specialty rhododendron nurseries has been so reduced. If azaleas and deciduous rhododendrons are acceptable, there are several true oranges in both species such as calendulaceum and hybrids.

I would definitely go with a deciduous azalea if able to do so. There are many bright, vibrant orange varieties that will suit.

A good orange is by far the most elusive color in elepidote rhododendrons. It seems like many attempts to breed one just create flowers that are a mix of pink and yellow: Hirsutum.info -- Rhododendron Hybrids/cultivars: 'Darigold', In person, with my own eyes, the closest I've seen is 'Medusa'. If you mean -10F, it isn't nearly hardy enough. It is barely hardy for me and would probably be zoned 7b or even 8a. Years ago, Jerry at Rarefind told me one got to be 6' on the coast of New Jersey but was killed by one of the cold winters of prior decades...presumably the late 70s. (the stretch from the early 60s to 1977 was very mild) After many hours of perusing hirsutum.org and other rhodie picture sites, the best I've seen in pictures is this one. Hirsutum.info -- Rhododendron Hybrids/cultivars: 'Nancy Evans x horaeum' Probably also only zn 7 hardy, and only available in Europe, if at all. However I was at a breeder's meeting at Rarefind and one hybridizer in the Northeast USA has produced something that at least looks almost as good in pictures, using a 1st gen offspring of 'Nancy Evans'. If you look at her progeny (and in the case of that cultivar, very appropriate to say 'her' because it produces no pollen!) she tends to produce colorful offspring, unlike some yellows whose color genes seem recessive and whose offspring are mostly pinkish or cream colored. No idea when that plant will ever been available, but it's on my top 10 sought after list, at least until I can see it in person and confirm it really looks that good.

Unfortunately there seems to be several major problems evaluating colors of red/yellow/orange rhodies: people or wholesalers either deliberately skew the color hue or saturation slightly using digital tools, or some cultivars produce better colors in climates with cloudy cool springs. Some of the colors in the Dutch book Rhododendron Portraits are totally implausible for a given cultivar, in my opinion at least. Either plants color much better there in a wet maritime climate or there's some semi-deliberate misrepresentation. But even my own 'Viscy' looks yellowish some years, closer to orangish others, and almost peachy-pink other years. There's no definitively discernable pattern or reason why, although it does seem cooler springs make better colors. ('Medusa' is also variable, it was an excellent orange one year but pinkish the next)

Anyhow based on plants in my own garden, if I could grow only one elepidote rhodie conventionally rated zn 6 and considered "orange", it would be 'Viscy'. I have 'Autumn Gold', 'Old Copper', 'Mary Belle', 'Mary Garrison' which are all nice plants, but there's something special about 'Viscy' to me. I go for more of a species look. Some people might not agree with me, who want a more hybridized looking plant.

Look at the deciduous Northern Lights azalea "Mandarin".

I have a very large one that's quite old now - had it since they were first introduced. It would laugh at -10 temps. We get sustained temps of minus 20 to 30 here, and a rare -50 with a wind chill. Hasn't missed a beat in all these years.