I know you already got an answer, but as another native English speaker, I can confirm that we say "The color orange" and not "The orange color."
The second one would only be used if you were talking about a very specific color of orange. Maybe if there were buckets of paint and you were talking about an orange bucket, you might say, "The orange color is okay, but that purple color is even better." But talking about the color orange in general, definitely "the color orange."
Actually, "arancio" is the orange tree, the fruit is "arancia" (plural: "arance").
A hint: often fruit tree are masculine such as the related fruit is feminine. E.g.: melo/mela; pero/pera; ciliegio/ciliegia; arancio/arancia; pruno/prugna; nocciòlo/nocciola; banano/banana... but: limone and fico are always masculine!!! (pay more attention on "fico"!) ;)
The color orange and the orange color would not be used in the same way. You might say the orange color in that dress is bright. But if you wanted to talk about the color orange in general, you would say The color orange. This article has it on its title.
I am sure if you perform a search where you are, you'll find plenty of hits for The colour orange.
As for your need for British spelling, I know that Duo accepts British spellings and terms, although I suspect it's always a struggle. Duolingo is an American company headquartered in Pennsylvania. They speak American English. They have never said anything but that. What is displayed as an answer will always be in American English. Get over yourself.
The base form of an adjective that you learn is generally the masculine one ending in o. If the first form that you think of doesn't end in o, then you should know that there are going to be differences. Both adjectives and nouns ending in e have to be different, because e is the normal ending for femmine plural nouns and their adjectives. Arancione has only two forms. Arancione is for either masculine or feminine nouns in the singular. Arancioni is used for both masculine and feminine nouns in the plural. A similar thing happens with nouns ending in e. They all form plurals with i, whether they are masculine and feminine. It can be misleading for learners who haven't learned all the genders of the nouns they know. It can mess up that perfect Italian system of having matching endings for the article, noun and adjective and/or make you think a feminine noun is masculine.
Things posted here are not read by Duo staff members. This is a user forum. If you want new answers to be accepted, you have to report them using the report button and choosing my answer should have been accepted. You certainly know better than I how often that spelling is accepted on Duo, but I know it is some places. But the fact that each exercise has its own database of accepted answers and the exercises are all written by the American staff, you may have to make this same request for many exercises. Changing it in one, won't affect the others.